diff --git a/src/archive.cc b/src/archive.cc
index 4a6211e00..9039ad7db 100644
--- a/src/archive.cc
+++ b/src/archive.cc
@@ -49,7 +49,7 @@ static void writeString(const string & s, DumpSink & sink)
 static void dump(const string & path, DumpSink & sink);
 
 
-static void dumpEntries(const string & path, DumpSink & sink)
+static void dumpEntries(const Path & path, DumpSink & sink)
 {
     DIR * dir = opendir(path.c_str());
     if (!dir) throw SysError("opening directory " + path);
@@ -82,7 +82,7 @@ static void dumpEntries(const string & path, DumpSink & sink)
 }
 
 
-static void dumpContents(const string & path, unsigned int size, 
+static void dumpContents(const Path & path, unsigned int size, 
     DumpSink & sink)
 {
     writeString("contents", sink);
@@ -110,7 +110,7 @@ static void dumpContents(const string & path, unsigned int size,
 }
 
 
-static void dump(const string & path, DumpSink & sink)
+static void dump(const Path & path, DumpSink & sink)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
@@ -150,7 +150,7 @@ static void dump(const string & path, DumpSink & sink)
 }
 
 
-void dumpPath(const string & path, DumpSink & sink)
+void dumpPath(const Path & path, DumpSink & sink)
 {
     writeString(archiveVersion1, sink);
     dump(path, sink);
@@ -207,10 +207,10 @@ static void skipGeneric(RestoreSource & source)
 }
 
 
-static void restore(const string & path, RestoreSource & source);
+static void restore(const Path & path, RestoreSource & source);
 
 
-static void restoreEntry(const string & path, RestoreSource & source)
+static void restoreEntry(const Path & path, RestoreSource & source)
 {
     string s, name;
 
@@ -235,7 +235,7 @@ static void restoreEntry(const string & path, RestoreSource & source)
 }
 
 
-static void restoreContents(int fd, const string & path, RestoreSource & source)
+static void restoreContents(int fd, const Path & path, RestoreSource & source)
 {
     unsigned int size = readInt(source);
     unsigned int left = size;
@@ -254,7 +254,7 @@ static void restoreContents(int fd, const string & path, RestoreSource & source)
 }
 
 
-static void restore(const string & path, RestoreSource & source)
+static void restore(const Path & path, RestoreSource & source)
 {
     string s;
 
@@ -331,7 +331,7 @@ static void restore(const string & path, RestoreSource & source)
 }
 
 
-void restorePath(const string & path, RestoreSource & source)
+void restorePath(const Path & path, RestoreSource & source)
 {
     if (readString(source) != archiveVersion1)
         throw badArchive("expected Nix archive");
diff --git a/src/archive.hh b/src/archive.hh
index e6006e454..67e236668 100644
--- a/src/archive.hh
+++ b/src/archive.hh
@@ -1,6 +1,6 @@
 #include <string>
 
-using namespace std;
+#include "util.hh"
 
 
 /* dumpPath creates a Nix archive of the specified path.  The format
@@ -45,7 +45,7 @@ struct DumpSink
     virtual void operator () (const unsigned char * data, unsigned int len) = 0;
 };
 
-void dumpPath(const string & path, DumpSink & sink);
+void dumpPath(const Path & path, DumpSink & sink);
 
 
 struct RestoreSource
@@ -57,4 +57,4 @@ struct RestoreSource
     virtual void operator () (unsigned char * data, unsigned int len) = 0;
 };
 
-void restorePath(const string & path, RestoreSource & source);
+void restorePath(const Path & path, RestoreSource & source);
diff --git a/src/db.cc b/src/db.cc
index 61ecb203a..ffb1999fe 100644
--- a/src/db.cc
+++ b/src/db.cc
@@ -258,6 +258,8 @@ void Database::delPair(const Transaction & txn, TableId table,
         Db * db = getDb(table);
         Dbt kt((void *) key.c_str(), key.length());
         db->del(txn.txn, &kt, 0);
+        /* Non-existence of a pair with the given key is not an
+           error. */
     } catch (DbException e) { rethrow(e); }
 }
 
diff --git a/src/dotgraph.cc b/src/dotgraph.cc
index f9ff51b90..514fda325 100644
--- a/src/dotgraph.cc
+++ b/src/dotgraph.cc
@@ -45,24 +45,24 @@ static string symbolicName(const string & path)
 }
 
 
-string pathLabel(const FSId & id, const string & path)
+string pathLabel(const Path & nePath, const string & elemPath)
 {
-    return (string) id + "-" + path;
+    return (string) nePath + "-" + elemPath;
 }
 
 
-void printClosure(const FSId & id, const NixExpr & fs)
+void printClosure(const Path & nePath, const NixExpr & fs)
 {
-    Strings workList(fs.closure.roots.begin(), fs.closure.roots.end());
-    StringSet doneSet;
+    PathSet workList(fs.closure.roots);
+    PathSet doneSet;
 
-    for (Strings::iterator i = workList.begin(); i != workList.end(); i++) {
-	cout << makeEdge(pathLabel(id, *i), id);
+    for (PathSet::iterator i = workList.begin(); i != workList.end(); i++) {
+	cout << makeEdge(pathLabel(nePath, *i), nePath);
     }
 
     while (!workList.empty()) {
-	string path = workList.front();
-	workList.pop_front();
+	Path path = *(workList.begin());
+	workList.erase(path);
 
 	if (doneSet.find(path) == doneSet.end()) {
 	    doneSet.insert(path);
@@ -74,41 +74,41 @@ void printClosure(const FSId & id, const NixExpr & fs)
 	    for (StringSet::const_iterator i = elem->second.refs.begin();
 		 i != elem->second.refs.end(); i++)
 	    {
-		workList.push_back(*i);
-		cout << makeEdge(pathLabel(id, *i), pathLabel(id, path));
+		workList.insert(*i);
+		cout << makeEdge(pathLabel(nePath, *i), pathLabel(nePath, path));
 	    }
 
-	    cout << makeNode(pathLabel(id, path), 
+	    cout << makeNode(pathLabel(nePath, path), 
 		symbolicName(path), "#ff0000");
 	}
     }
 }
 
 
-void printDotGraph(const FSIds & roots)
+void printDotGraph(const PathSet & roots)
 {
-    FSIds workList(roots.begin(), roots.end());
-    FSIdSet doneSet;
+    PathSet workList(roots);
+    PathSet doneSet;
             
     cout << "digraph G {\n";
 
     while (!workList.empty()) {
-	FSId id = workList.front();
-	workList.pop_front();
+	Path nePath = *(workList.begin());
+	workList.erase(nePath);
 
-	if (doneSet.find(id) == doneSet.end()) {
-	    doneSet.insert(id);
+	if (doneSet.find(nePath) == doneSet.end()) {
+	    doneSet.insert(nePath);
                     
-	    NixExpr ne = parseNixExpr(termFromId(id));
+	    NixExpr ne = parseNixExpr(termFromPath(nePath));
 
 	    string label, colour;
                     
 	    if (ne.type == NixExpr::neDerivation) {
-		for (FSIdSet::iterator i = ne.derivation.inputs.begin();
+		for (PathSet::iterator i = ne.derivation.inputs.begin();
 		     i != ne.derivation.inputs.end(); i++)
 		{
-		    workList.push_back(*i);
-		    cout << makeEdge(*i, id);
+		    workList.insert(*i);
+		    cout << makeEdge(*i, nePath);
 		}
 
 		label = "derivation";
@@ -121,12 +121,12 @@ void printDotGraph(const FSIds & roots)
 	    else if (ne.type == NixExpr::neClosure) {
 		label = "<closure>";
 		colour = "#00ffff";
-		printClosure(id, ne);
+		printClosure(nePath, ne);
 	    }
 
 	    else abort();
 
-	    cout << makeNode(id, label, colour);
+	    cout << makeNode(nePath, label, colour);
 	}
     }
 
diff --git a/src/dotgraph.hh b/src/dotgraph.hh
index e1678da96..a5c5248f1 100644
--- a/src/dotgraph.hh
+++ b/src/dotgraph.hh
@@ -3,6 +3,6 @@
 
 #include "expr.hh"
 
-void printDotGraph(const FSIds & roots);
+void printDotGraph(const PathSet & roots);
 
 #endif /* !__DOTGRAPH_H */
diff --git a/src/expr.cc b/src/expr.cc
index 7d79d1654..2ed3e678b 100644
--- a/src/expr.cc
+++ b/src/expr.cc
@@ -22,37 +22,33 @@ Hash hashTerm(ATerm t)
 }
 
 
-ATerm termFromId(const FSId & id)
+ATerm termFromPath(const Path & path)
 {
-    string path = expandId(id);
     ATerm t = ATreadFromNamedFile(path.c_str());
     if (!t) throw Error(format("cannot read aterm from `%1%'") % path);
     return t;
 }
 
 
-FSId writeTerm(ATerm t, const string & suffix, FSId id)
+Path writeTerm(ATerm t, const string & suffix)
 {
-    /* By default, the id of a term is its hash. */
-    if (id == FSId()) id = hashTerm(t);
+    /* The id of a term is its hash. */
+    Hash h = hashTerm(t);
 
-    string path = canonPath(nixStore + "/" + 
-        (string) id + suffix + ".nix");
+    Path path = canonPath(nixStore + "/" + 
+        (string) h + suffix + ".nix");
     if (!ATwriteToNamedTextFile(t, path.c_str()))
         throw Error(format("cannot write aterm %1%") % path);
 
-//     debug(format("written term %1% = %2%") % (string) id %
-//         printTerm(t));
-
     Transaction txn(nixDB);
-    registerPath(txn, path, id);
+    registerValidPath(txn, path);
     txn.commit();
 
-    return id;
+    return path;
 }
 
 
-static void parsePaths(ATermList paths, StringSet & out)
+static void parsePaths(ATermList paths, PathSet & out)
 {
     while (!ATisEmpty(paths)) {
         char * s;
@@ -70,19 +66,19 @@ static void checkClosure(const Closure & closure)
     if (closure.elems.size() == 0)
         throw Error("empty closure");
 
-    StringSet decl;
+    PathSet decl;
     for (ClosureElems::const_iterator i = closure.elems.begin();
          i != closure.elems.end(); i++)
         decl.insert(i->first);
     
-    for (StringSet::const_iterator i = closure.roots.begin();
+    for (PathSet::const_iterator i = closure.roots.begin();
          i != closure.roots.end(); i++)
         if (decl.find(*i) == decl.end())
             throw Error(format("undefined root path `%1%'") % *i);
     
     for (ClosureElems::const_iterator i = closure.elems.begin();
          i != closure.elems.end(); i++)
-        for (StringSet::const_iterator j = i->second.refs.begin();
+        for (PathSet::const_iterator j = i->second.refs.begin();
              j != i->second.refs.end(); j++)
             if (decl.find(*j) == decl.end())
                 throw Error(
@@ -102,13 +98,12 @@ static bool parseClosure(ATerm t, Closure & closure)
     parsePaths(roots, closure.roots);
 
     while (!ATisEmpty(elems)) {
-        char * s1, * s2;
+        char * s1;
         ATermList refs;
         ATerm t = ATgetFirst(elems);
-        if (!ATmatch(t, "(<str>, <str>, [<list>])", &s1, &s2, &refs))
+        if (!ATmatch(t, "(<str>, [<list>])", &s1, &refs))
             throw badTerm("not a closure element", t);
         ClosureElem elem;
-        elem.id = parseHash(s2);
         parsePaths(refs, elem.refs);
         closure.elems[s1] = elem;
         elems = ATgetNext(elems);
@@ -135,23 +130,8 @@ static bool parseDerivation(ATerm t, Derivation & derivation)
         args = ATempty;
     }
 
-    while (!ATisEmpty(outs)) {
-        char * s1, * s2;
-        ATerm t = ATgetFirst(outs);
-        if (!ATmatch(t, "(<str>, <str>)", &s1, &s2))
-            throw badTerm("not a derivation output", t);
-        derivation.outputs[s1] = parseHash(s2);
-        outs = ATgetNext(outs);
-    }
-
-    while (!ATisEmpty(ins)) {
-        char * s;
-        ATerm t = ATgetFirst(ins);
-        if (!ATmatch(t, "<str>", &s))
-            throw badTerm("not an id", t);
-        derivation.inputs.insert(parseHash(s));
-        ins = ATgetNext(ins);
-    }
+    parsePaths(outs, derivation.outputs);
+    parsePaths(ins, derivation.inputs);
 
     derivation.builder = builder;
     derivation.platform = platform;
@@ -190,10 +170,10 @@ NixExpr parseNixExpr(ATerm t)
 }
 
 
-static ATermList unparsePaths(const StringSet & paths)
+static ATermList unparsePaths(const PathSet & paths)
 {
     ATermList l = ATempty;
-    for (StringSet::const_iterator i = paths.begin();
+    for (PathSet::const_iterator i = paths.begin();
          i != paths.end(); i++)
         l = ATinsert(l, ATmake("<str>", i->c_str()));
     return ATreverse(l);
@@ -208,9 +188,8 @@ static ATerm unparseClosure(const Closure & closure)
     for (ClosureElems::const_iterator i = closure.elems.begin();
          i != closure.elems.end(); i++)
         elems = ATinsert(elems,
-            ATmake("(<str>, <str>, <term>)",
+            ATmake("(<str>, <term>)",
                 i->first.c_str(),
-                ((string) i->second.id).c_str(),
                 unparsePaths(i->second.refs)));
 
     return ATmake("Closure(<term>, <term>)", roots, elems);
@@ -219,18 +198,6 @@ static ATerm unparseClosure(const Closure & closure)
 
 static ATerm unparseDerivation(const Derivation & derivation)
 {
-    ATermList outs = ATempty;
-    for (DerivationOutputs::const_iterator i = derivation.outputs.begin();
-         i != derivation.outputs.end(); i++)
-        outs = ATinsert(outs,
-            ATmake("(<str>, <str>)", 
-                i->first.c_str(), ((string) i->second).c_str()));
-    
-    ATermList ins = ATempty;
-    for (FSIdSet::const_iterator i = derivation.inputs.begin();
-         i != derivation.inputs.end(); i++)
-        ins = ATinsert(ins, ATmake("<str>", ((string) *i).c_str()));
-
     ATermList args = ATempty;
     for (Strings::const_iterator i = derivation.args.begin();
          i != derivation.args.end(); i++)
@@ -244,8 +211,8 @@ static ATerm unparseDerivation(const Derivation & derivation)
                 i->first.c_str(), i->second.c_str()));
 
     return ATmake("Derive(<term>, <term>, <str>, <str>, <term>, <term>)",
-        ATreverse(outs),
-        ATreverse(ins),
+        unparsePaths(derivation.outputs),
+        unparsePaths(derivation.inputs),
         derivation.platform.c_str(),
         derivation.builder.c_str(),
         ATreverse(args),
diff --git a/src/expr.hh b/src/expr.hh
index 2a6a06a0a..b34564322 100644
--- a/src/expr.hh
+++ b/src/expr.hh
@@ -10,31 +10,27 @@ extern "C" {
 
 /* Abstract syntax of Nix expressions. */
 
-typedef list<FSId> FSIds;
-
 struct ClosureElem
 {
-    FSId id;
-    StringSet refs;
+    PathSet refs;
 };
 
-typedef map<string, ClosureElem> ClosureElems;
+typedef map<Path, ClosureElem> ClosureElems;
 
 struct Closure
 {
-    StringSet roots;
+    PathSet roots;
     ClosureElems elems;
 };
 
-typedef map<string, FSId> DerivationOutputs;
 typedef map<string, string> StringPairs;
 
 struct Derivation
 {
-    DerivationOutputs outputs;
-    FSIdSet inputs;
+    PathSet outputs;
+    PathSet inputs; /* Nix expressions, not actual inputs */
     string platform;
-    string builder;
+    Path builder;
     Strings args;
     StringPairs env;
 };
@@ -57,11 +53,11 @@ Error badTerm(const format & f, ATerm t);
 /* Hash an aterm. */
 Hash hashTerm(ATerm t);
 
-/* Read an aterm from disk, given its id. */
-ATerm termFromId(const FSId & id);
+/* Read an aterm from disk. */
+ATerm termFromPath(const Path & path);
 
-/* Write an aterm to the Nix store directory, and return its hash. */
-FSId writeTerm(ATerm t, const string & suffix, FSId id = FSId());
+/* Write an aterm to the Nix store directory, and return its path. */
+Path writeTerm(ATerm t, const string & suffix);
 
 /* Parse a Nix expression. */
 NixExpr parseNixExpr(ATerm t);
diff --git a/src/fix.cc b/src/fix.cc
index 66438464c..71fd06877 100644
--- a/src/fix.cc
+++ b/src/fix.cc
@@ -9,12 +9,12 @@
 typedef ATerm Expr;
 
 typedef map<ATerm, ATerm> NormalForms;
-typedef map<FSId, Strings> PkgPaths;
-typedef map<FSId, Hash> PkgHashes;
+typedef map<Path, PathSet> PkgPaths;
+typedef map<Path, Hash> PkgHashes;
 
 struct EvalState 
 {
-    Strings searchDirs;
+    Paths searchDirs;
     NormalForms normalForms;
     PkgPaths pkgPaths;
     PkgHashes pkgHashes; /* normalised package hashes */
@@ -28,18 +28,18 @@ struct EvalState
 };
 
 
-static Expr evalFile(EvalState & state, string fileName);
+static Expr evalFile(EvalState & state, const Path & path);
 static Expr evalExpr(EvalState & state, Expr e);
 
 
-static string searchPath(const Strings & searchDirs, string relPath)
+static Path searchPath(const Paths & searchDirs, const Path & relPath)
 {
     if (string(relPath, 0, 1) == "/") return relPath;
 
-    for (Strings::const_iterator i = searchDirs.begin();
+    for (Paths::const_iterator i = searchDirs.begin();
          i != searchDirs.end(); i++)
     {
-        string path = *i + "/" + relPath;
+        Path path = *i + "/" + relPath;
         if (pathExists(path)) return path;
     }
 
@@ -121,14 +121,14 @@ static Expr substExprMany(ATermList formals, ATermList args, Expr body)
 }
 
 
-static Strings nixExprPathsCached(EvalState & state, const FSId & id)
+static PathSet nixExprRootsCached(EvalState & state, const Path & nePath)
 {
-    PkgPaths::iterator i = state.pkgPaths.find(id);
+    PkgPaths::iterator i = state.pkgPaths.find(nePath);
     if (i != state.pkgPaths.end())
         return i->second;
     else {
-        Strings paths = nixExprPaths(id);
-        state.pkgPaths[id] = paths;
+        PathSet paths = nixExprRoots(nePath);
+        state.pkgPaths[nePath] = paths;
         return paths;
     }
 }
@@ -137,13 +137,13 @@ static Strings nixExprPathsCached(EvalState & state, const FSId & id)
 static Hash hashPackage(EvalState & state, NixExpr ne)
 {
     if (ne.type == NixExpr::neDerivation) {
-	FSIdSet inputs2;
-        for (FSIdSet::iterator i = ne.derivation.inputs.begin();
+	PathSet inputs2;
+        for (PathSet::iterator i = ne.derivation.inputs.begin();
              i != ne.derivation.inputs.end(); i++)
         {
             PkgHashes::iterator j = state.pkgHashes.find(*i);
             if (j == state.pkgHashes.end())
-                throw Error(format("unknown package id %1%") % (string) *i);
+                throw Error(format("don't know expression `%1%'") % (string) *i);
             inputs2.insert(j->second);
         }
 	ne.derivation.inputs = inputs2;
@@ -156,12 +156,12 @@ static string processBinding(EvalState & state, Expr e, NixExpr & ne)
 {
     char * s1;
 
-    if (ATmatch(e, "FSId(<str>)", &s1)) {
-        FSId id = parseHash(s1);
-        Strings paths = nixExprPathsCached(state, id);
+    if (ATmatch(e, "NixExpr(<str>)", &s1)) {
+        Path nePath(s1);
+        PathSet paths = nixExprRootsCached(state, nePath);
         if (paths.size() != 1) abort();
-        string path = *(paths.begin());
-        ne.derivation.inputs.insert(id);
+        Path path = *(paths.begin());
+        ne.derivation.inputs.insert(nePath);
         return path;
     }
     
@@ -200,14 +200,14 @@ static Expr evalExpr2(EvalState & state, Expr e)
         ATmatch(e, "True") ||
         ATmatch(e, "False") ||
         ATmatch(e, "Function([<list>], <term>)", &e1, &e2) ||
-        ATmatch(e, "FSId(<str>)", &s1))
+        ATmatch(e, "NixExpr(<str>)", &s1))
         return e;
 
     try {
         Hash pkgHash = hashPackage(state, parseNixExpr(e));
-        FSId pkgId = writeTerm(e, "");
-        state.pkgHashes[pkgId] = pkgHash;
-        return ATmake("FSId(<str>)", ((string) pkgId).c_str());
+        Path pkgPath = writeTerm(e, "");
+        state.pkgHashes[pkgPath] = pkgHash;
+        return ATmake("NixExpr(<str>)", pkgPath.c_str());
     } catch (...) { /* !!! catch parse errors only */
     }
 
@@ -254,32 +254,29 @@ static Expr evalExpr2(EvalState & state, Expr e)
 
     /* Fix inclusion. */
     if (ATmatch(e, "IncludeFix(<str>)", &s1)) {
-        string fileName(s1);
+        Path fileName(s1);
         return evalFile(state, s1);
     }
 
     /* Relative files. */
     if (ATmatch(e, "Relative(<str>)", &s1)) {
-        string srcPath = searchPath(state.searchDirs, s1);
-        string dstPath;
-        FSId id;
-        addToStore(srcPath, dstPath, id, true);
+        Path srcPath = searchPath(state.searchDirs, s1);
+        Path dstPath = addToStore(srcPath);
 
         ClosureElem elem;
-        elem.id = id;
         NixExpr ne;
         ne.type = NixExpr::neClosure;
         ne.closure.roots.insert(dstPath);
         ne.closure.elems[dstPath] = elem;
 
         Hash pkgHash = hashPackage(state, ne);
-        FSId pkgId = writeTerm(unparseNixExpr(ne), "");
-        state.pkgHashes[pkgId] = pkgHash;
+        Path pkgPath = writeTerm(unparseNixExpr(ne), "");
+        state.pkgHashes[pkgPath] = pkgHash;
 
-        msg(lvlChatty, format("copied `%1%' -> %2%")
-            % srcPath % (string) pkgId);
+        msg(lvlChatty, format("copied `%1%' -> closure `%2%'")
+            % srcPath % pkgPath);
 
-        return ATmake("FSId(<str>)", ((string) pkgId).c_str());
+        return ATmake("NixExpr(<str>)", pkgPath.c_str());
     }
 
     /* Packages are transformed into Nix derivation expressions. */
@@ -302,8 +299,8 @@ static Expr evalExpr2(EvalState & state, Expr e)
         ne.type = NixExpr::neDerivation;
         ne.derivation.platform = SYSTEM;
         string name;
-        FSId outId;
-        bool outIdGiven = false;
+        Hash outHash;
+        bool outHashGiven = false;
         bnds = ATempty;
 
         for (map<string, ATerm>::iterator it = bndMap.begin();
@@ -331,8 +328,8 @@ static Expr evalExpr2(EvalState & state, Expr e)
                 if (key == "build") ne.derivation.builder = s;
                 if (key == "name") name = s;
                 if (key == "id") { 
-                    outId = parseHash(s);
-                    outIdGiven = true;
+                    outHash = parseHash(s);
+                    outHashGiven = true;
                 }
             }
 
@@ -348,23 +345,23 @@ static Expr evalExpr2(EvalState & state, Expr e)
         
         /* Hash the Nix expression with no outputs to produce a
            unique but deterministic path name for this package. */
-        if (!outIdGiven) outId = hashPackage(state, ne);
-        string outPath = 
-            canonPath(nixStore + "/" + ((string) outId).c_str() + "-" + name);
+        if (!outHashGiven) outHash = hashPackage(state, ne);
+        Path outPath = 
+            canonPath(nixStore + "/" + ((string) outHash).c_str() + "-" + name);
         ne.derivation.env["out"] = outPath;
-        ne.derivation.outputs[outPath] = outId;
+        ne.derivation.outputs.insert(outPath);
 
         /* Write the resulting term into the Nix store directory. */
-        Hash pkgHash = outIdGiven
-            ? hashString((string) outId + outPath)
+        Hash pkgHash = outHashGiven
+            ? hashString((string) outHash + outPath)
             : hashPackage(state, ne);
-        FSId pkgId = writeTerm(unparseNixExpr(ne), "-d-" + name);
-        state.pkgHashes[pkgId] = pkgHash;
+        Path pkgPath = writeTerm(unparseNixExpr(ne), "-d-" + name);
+        state.pkgHashes[pkgPath] = pkgHash;
 
-        msg(lvlChatty, format("instantiated `%1%' -> %2%")
-            % name % (string) pkgId);
+        msg(lvlChatty, format("instantiated `%1%' -> `%2%'")
+            % name % pkgPath);
 
-        return ATmake("FSId(<str>)", ((string) pkgId).c_str());
+        return ATmake("NixExpr(<str>)", pkgPath.c_str());
     }
 
     /* BaseName primitive function. */
@@ -401,9 +398,9 @@ static Expr evalExpr(EvalState & state, Expr e)
 }
 
 
-static Expr evalFile(EvalState & state, string relPath)
+static Expr evalFile(EvalState & state, const Path & relPath)
 {
-    string path = searchPath(state.searchDirs, relPath);
+    Path path = searchPath(state.searchDirs, relPath);
     Nest nest(lvlTalkative, format("evaluating file `%1%'") % path);
     Expr e = ATreadFromNamedFile(path.c_str());
     if (!e) 
@@ -422,16 +419,16 @@ static Expr evalStdin(EvalState & state)
 }
 
 
-static void printFSId(EvalState & state, Expr e)
+static void printNixExpr(EvalState & state, Expr e)
 {
     ATermList es;
     char * s;
-    if (ATmatch(e, "FSId(<str>)", &s)) {
+    if (ATmatch(e, "NixExpr(<str>)", &s)) {
         cout << format("%1%\n") % s;
     } 
     else if (ATmatch(e, "[<list>]", &es)) {
         while (!ATisEmpty(es)) {
-            printFSId(state, evalExpr(state, ATgetFirst(es)));
+            printNixExpr(state, evalExpr(state, ATgetFirst(es)));
             es = ATgetNext(es);
         }
     }
@@ -472,14 +469,14 @@ void run(Strings args)
 
     if (readStdin) {
         Expr e = evalStdin(state);
-        printFSId(state, e);
+        printNixExpr(state, e);
     }
 
     for (Strings::iterator it = files.begin();
          it != files.end(); it++)
     {
         Expr e = evalFile(state, *it);
-        printFSId(state, e);
+        printNixExpr(state, e);
     }
 }
 
diff --git a/src/globals.cc b/src/globals.cc
index f21820f59..f34f8f102 100644
--- a/src/globals.cc
+++ b/src/globals.cc
@@ -5,8 +5,7 @@
 Database nixDB;
 
 
-TableId dbPath2Id;
-TableId dbId2Paths;
+TableId dbValidPaths;
 TableId dbSuccessors;
 TableId dbSubstitutes;
 
@@ -23,8 +22,7 @@ bool keepFailed = false;
 void openDB()
 {
     nixDB.open(nixDBPath);
-    dbPath2Id = nixDB.openTable("path2id");
-    dbId2Paths = nixDB.openTable("id2paths");
+    dbValidPaths = nixDB.openTable("validpaths");
     dbSuccessors = nixDB.openTable("successors");
     dbSubstitutes = nixDB.openTable("substitutes");
 }
diff --git a/src/globals.hh b/src/globals.hh
index b140f136c..0ea2fca09 100644
--- a/src/globals.hh
+++ b/src/globals.hh
@@ -13,42 +13,36 @@ extern Database nixDB;
 
 /* Database tables. */
 
-/* dbPath2Id :: Path -> FSId
 
-   Each pair (p, id) records that path $p$ contains an expansion of
-   $id$. */
-extern TableId dbPath2Id;
+/* dbValidPaths :: Path -> ()
+
+   The existence of a key $p$ indicates that path $p$ is valid (that
+   is, produced by a succesful build). */
+extern TableId dbValidPaths;
 
 
-/* dbId2Paths :: FSId -> [Path]
+/* dbSuccessors :: Path -> Path
 
-   A mapping from ids to lists of paths. */
-extern TableId dbId2Paths;
+   Each pair $(p_1, p_2)$ in this mapping records the fact that the
+   Nix expression stored at path $p_1$ has a successor expression
+   stored at path $p_2$.
 
-
-/* dbSuccessors :: FSId -> FSId
-
-   Each pair $(id_1, id_2)$ in this mapping records the fact that a
-   successor of a Nix expression stored in a file with identifier
-   $id_1$ is stored in a file with identifier $id_2$.
-
-   Note that a term $y$ is successor of $x$ iff there exists a
+   Note that a term $y$ is a successor of $x$ iff there exists a
    sequence of rewrite steps that rewrites $x$ into $y$.
 */
 extern TableId dbSuccessors;
 
 
-/* dbSubstitutes :: FSId -> [FSId]
+/* dbSubstitutes :: Path -> [Path]
 
-   Each pair $(id, [ids])$ tells Nix that it can realise any of the
-   Nix expressions referenced by the identifiers in $ids$ to
-   generate a path with identifier $id$.
+   Each pair $(p, [ps])$ tells Nix that it can realise any of the
+   Nix expressions stored at paths $ps$ to produce a path $p$.
 
    The main purpose of this is for distributed caching of derivates.
-   One system can compute a derivate with hash $h$ and put it on a
-   website (as a Nix archive), for instance, and then another system
-   can register a substitute for that derivate.  The substitute in
-   this case might be a Nix expression that fetches the Nix archive.
+   One system can compute a derivate and put it on a website (as a Nix
+   archive), for instance, and then another system can register a
+   substitute for that derivate.  The substitute in this case might be
+   a Nix expression that fetches the Nix archive.
 */
 extern TableId dbSubstitutes;
 
diff --git a/src/hash.cc b/src/hash.cc
index b59c4f214..752b26912 100644
--- a/src/hash.cc
+++ b/src/hash.cc
@@ -54,11 +54,11 @@ Hash parseHash(const string & s)
 {
     Hash hash;
     if (s.length() != Hash::hashSize * 2)
-        throw BadRefError("invalid hash: " + s);
+        throw Error(format("invalid hash `%1%'") % s);
     for (unsigned int i = 0; i < Hash::hashSize; i++) {
         string s2(s, i * 2, 2);
         if (!isxdigit(s2[0]) || !isxdigit(s2[1])) 
-            throw BadRefError("invalid hash: " + s);
+            throw Error(format("invalid hash `%1%'") % s);
         istringstream str(s2);
         int n;
         str >> hex >> n;
@@ -89,15 +89,15 @@ Hash hashString(const string & s)
 }
 
 
-Hash hashFile(const string & fileName)
+Hash hashFile(const Path & path)
 {
     Hash hash;
-    FILE * file = fopen(fileName.c_str(), "rb");
+    FILE * file = fopen(path.c_str(), "rb");
     if (!file)
-        throw SysError("file `" + fileName + "' does not exist");
+        throw SysError(format("file `%1%' does not exist") % path);
     int err = md5_stream(file, hash.hash);
     fclose(file);
-    if (err) throw SysError("cannot hash file " + fileName);
+    if (err) throw SysError(format("cannot hash file `%1%'") % path);
     return hash;
 }
 
@@ -113,7 +113,7 @@ struct HashSink : DumpSink
 };
 
 
-Hash hashPath(const string & path)
+Hash hashPath(const Path & path)
 {
     Hash hash;
     HashSink sink;
diff --git a/src/hash.hh b/src/hash.hh
index 387939e93..0062f987c 100644
--- a/src/hash.hh
+++ b/src/hash.hh
@@ -30,13 +30,6 @@ struct Hash
 };
 
 
-class BadRefError : public Error
-{
-public:
-    BadRefError(string _err) : Error(_err) { };
-};
-
-
 /* Parse a hexadecimal representation of a hash code. */
 Hash parseHash(const string & s);
 
@@ -47,12 +40,12 @@ bool isHash(const string & s);
 Hash hashString(const string & s);
 
 /* Compute the hash of the given file. */
-Hash hashFile(const string & fileName);
+Hash hashFile(const Path & path);
 
 /* Compute the hash of the given path.  The hash is defined as
    md5(dump(path)).
 */
-Hash hashPath(const string & path);
+Hash hashPath(const Path & path);
 
 
 #endif /* !__HASH_H */
diff --git a/src/nix-help.txt b/src/nix-help.txt
index be8ecf0a1..a51018ea1 100644
--- a/src/nix-help.txt
+++ b/src/nix-help.txt
@@ -19,16 +19,11 @@ Operations:
   --version: output version information
   --help: display help
 
-Source selection for --install, --dump:
-
-  --path / -p: by file name  !!! -> path
-
 Query flags:
 
   --list / -l: query the output paths (roots) of a Nix expression (default)
   --requisites / -r: print all paths necessary to realise expression
   --generators / -g: find expressions producing a subset of given ids
-  --expansion / -e: print a path containing id
   --graph: print a dot graph rooted at given ids
 
 Options:
diff --git a/src/nix.cc b/src/nix.cc
index 87553de2d..9907f5c74 100644
--- a/src/nix.cc
+++ b/src/nix.cc
@@ -11,9 +11,6 @@
 typedef void (* Operation) (Strings opFlags, Strings opArgs);
 
 
-static bool pathArgs = false;
-
-
 static void printHelp()
 {
     cout <<
@@ -24,16 +21,9 @@ static void printHelp()
 
 
 
-static FSId argToId(const string & arg)
+static Path checkPath(const Path & arg)
 {
-    if (!pathArgs)
-        return parseHash(arg);
-    else {
-        FSId id;
-        if (!queryPathId(arg, id))
-            throw Error(format("don't know id of `%1%'") % arg);
-        return id;
-    }
+    return arg; /* !!! check that arg is in the store */
 }
 
 
@@ -42,12 +32,12 @@ static void opInstall(Strings opFlags, Strings opArgs)
 {
     if (!opFlags.empty()) throw UsageError("unknown flag");
 
-    for (Strings::iterator it = opArgs.begin();
-         it != opArgs.end(); it++)
+    for (Strings::iterator i = opArgs.begin();
+         i != opArgs.end(); i++)
     {
-        FSId id = normaliseNixExpr(argToId(*it));
-        realiseClosure(id);
-        cout << format("%1%\n") % (string) id;
+        Path nfPath = normaliseNixExpr(checkPath(*i));
+        realiseClosure(nfPath);
+        cout << format("%1%\n") % (string) nfPath;
     }
 }
 
@@ -59,7 +49,7 @@ static void opDelete(Strings opFlags, Strings opArgs)
 
     for (Strings::iterator it = opArgs.begin();
          it != opArgs.end(); it++)
-        deleteFromStore(absPath(*it));
+        deleteFromStore(checkPath(*it));
 }
 
 
@@ -69,27 +59,21 @@ static void opAdd(Strings opFlags, Strings opArgs)
 {
     if (!opFlags.empty()) throw UsageError("unknown flag");
 
-    for (Strings::iterator it = opArgs.begin();
-         it != opArgs.end(); it++)
-    {
-        string path;
-        FSId id;
-        addToStore(*it, path, id);
-        cout << format("%1% %2%\n") % (string) id % path;
-    }
+    for (Strings::iterator i = opArgs.begin(); i != opArgs.end(); i++)
+        cout << format("%1%\n") % addToStore(*i);
 }
 
 
-FSId maybeNormalise(const FSId & id, bool normalise)
+Path maybeNormalise(const Path & ne, bool normalise)
 {
-    return normalise ? normaliseNixExpr(id) : id;
+    return normalise ? normaliseNixExpr(ne) : ne;
 }
 
 
 /* Perform various sorts of queries. */
 static void opQuery(Strings opFlags, Strings opArgs)
 {
-    enum { qList, qRequisites, qGenerators, qExpansion, qGraph 
+    enum { qList, qRequisites, qGenerators, qGraph 
     } query = qList;
     bool normalise = false;
     bool includeExprs = true;
@@ -100,7 +84,6 @@ static void opQuery(Strings opFlags, Strings opArgs)
         if (*i == "--list" || *i == "-l") query = qList;
         else if (*i == "--requisites" || *i == "-r") query = qRequisites;
         else if (*i == "--generators" || *i == "-g") query = qGenerators;
-        else if (*i == "--expansion" || *i == "-e") query = qExpansion;
         else if (*i == "--graph") query = qGraph;
         else if (*i == "--normalise" || *i == "-n") normalise = true;
         else if (*i == "--exclude-exprs") includeExprs = false;
@@ -110,12 +93,12 @@ static void opQuery(Strings opFlags, Strings opArgs)
     switch (query) {
         
         case qList: {
-            StringSet paths;
+            PathSet paths;
             for (Strings::iterator i = opArgs.begin();
                  i != opArgs.end(); i++)
             {
-                Strings paths2 = nixExprPaths(
-                    maybeNormalise(argToId(*i), normalise));
+                StringSet paths2 = nixExprRoots(
+                    maybeNormalise(checkPath(*i), normalise));
                 paths.insert(paths2.begin(), paths2.end());
             }
             for (StringSet::iterator i = paths.begin(); 
@@ -129,8 +112,8 @@ static void opQuery(Strings opFlags, Strings opArgs)
             for (Strings::iterator i = opArgs.begin();
                  i != opArgs.end(); i++)
             {
-                Strings paths2 = nixExprRequisites(
-                    maybeNormalise(argToId(*i), normalise),
+                StringSet paths2 = nixExprRequisites(
+                    maybeNormalise(checkPath(*i), normalise),
                     includeExprs, includeSuccessors);
                 paths.insert(paths2.begin(), paths2.end());
             }
@@ -140,11 +123,12 @@ static void opQuery(Strings opFlags, Strings opArgs)
             break;
         }
 
+#if 0
         case qGenerators: {
             FSIds outIds;
             for (Strings::iterator i = opArgs.begin();
                  i != opArgs.end(); i++)
-                outIds.push_back(argToId(*i));
+                outIds.push_back(checkPath(*i));
 
             FSIds genIds = findGenerators(outIds);
 
@@ -153,21 +137,13 @@ static void opQuery(Strings opFlags, Strings opArgs)
                 cout << format("%s\n") % expandId(*i);
             break;
         }
-
-        case qExpansion: {
-            for (Strings::iterator i = opArgs.begin();
-                 i != opArgs.end(); i++)
-                /* !!! should not use substitutes; this is a query,
-                   it should not have side-effects */
-                cout << format("%s\n") % expandId(parseHash(*i));
-            break;
-        }
+#endif
 
         case qGraph: {
-            FSIds roots;
+            PathSet roots;
             for (Strings::iterator i = opArgs.begin();
                  i != opArgs.end(); i++)
-                roots.push_back(maybeNormalise(argToId(*i), normalise));
+                roots.insert(maybeNormalise(checkPath(*i), normalise));
 	    printDotGraph(roots);
             break;
         }
@@ -187,9 +163,9 @@ static void opSuccessor(Strings opFlags, Strings opArgs)
     for (Strings::iterator i = opArgs.begin();
          i != opArgs.end(); )
     {
-        FSId id1 = parseHash(*i++);
-        FSId id2 = parseHash(*i++);
-        registerSuccessor(txn, id1, id2);
+        Path path1 = checkPath(*i++);
+        Path path2 = checkPath(*i++);
+        registerSuccessor(txn, path1, path2);
     }
     txn.commit();
 }
@@ -203,8 +179,8 @@ static void opSubstitute(Strings opFlags, Strings opArgs)
     for (Strings::iterator i = opArgs.begin();
          i != opArgs.end(); )
     {
-        FSId src = parseHash(*i++);
-        FSId sub = parseHash(*i++);
+        Path src = checkPath(*i++);
+        Path sub = checkPath(*i++);
         registerSubstitute(src, sub);
     }
 }
@@ -229,9 +205,7 @@ static void opDump(Strings opFlags, Strings opArgs)
     if (opArgs.size() != 1) throw UsageError("only one argument allowed");
 
     StdoutSink sink;
-    string arg = *opArgs.begin();
-    string path = pathArgs ? arg : expandId(parseHash(arg));
-
+    string path = *opArgs.begin();
     dumpPath(path, sink);
 }
 
@@ -311,8 +285,6 @@ void run(Strings args)
             op = opInit;
         else if (arg == "--verify")
             op = opVerify;
-        else if (arg == "--path" || arg == "-p")
-            pathArgs = true;
         else if (arg == "--verbose" || arg == "-v")
             verbosity = (Verbosity) ((int) verbosity + 1);
         else if (arg == "--keep-failed" || arg == "-K")
diff --git a/src/normalise.cc b/src/normalise.cc
index 994a07df1..834276f1b 100644
--- a/src/normalise.cc
+++ b/src/normalise.cc
@@ -9,47 +9,131 @@
 
 
 void registerSuccessor(const Transaction & txn,
-    const FSId & id1, const FSId & id2)
+    const Path & path1, const Path & path2)
 {
-    nixDB.setString(txn, dbSuccessors, id1, id2);
+    nixDB.setString(txn, dbSuccessors, path1, path2);
 }
 
 
-static FSId useSuccessor(const FSId & id)
+static Path useSuccessor(const Path & path)
 {
-    string idSucc;
-    if (nixDB.queryString(noTxn, dbSuccessors, id, idSucc)) {
-        debug(format("successor %1% -> %2%") % (string) id % idSucc);
-        return parseHash(idSucc);
+    string pathSucc;
+    if (nixDB.queryString(noTxn, dbSuccessors, path, pathSucc)) {
+        debug(format("successor %1% -> %2%") % (string) path % pathSucc);
+        return pathSucc;
     } else
-        return id;
+        return path;
 }
 
 
-Strings pathsFromOutputs(const DerivationOutputs & ps)
+#if 0
+/* Return a path whose contents have the given hash.  If target is
+   not empty, ensure that such a path is realised in target (if
+   necessary by copying from another location).  If prefix is not
+   empty, only return a path that is an descendent of prefix. */
+
+string expandId(const FSId & id, const string & target = "",
+    const string & prefix = "/", FSIdSet pending = FSIdSet(),
+    bool ignoreSubstitutes = false)
 {
-    Strings ss;
-    for (DerivationOutputs::const_iterator i = ps.begin();
-         i != ps.end(); i++)
-        ss.push_back(i->first);
-    return ss;
+    xxx
 }
 
 
-FSId normaliseNixExpr(FSId id, FSIdSet pending)
+string expandId(const FSId & id, const string & target,
+    const string & prefix, FSIdSet pending, bool ignoreSubstitutes)
+{
+    Nest nest(lvlDebug, format("expanding %1%") % (string) id);
+
+    Strings paths;
+
+    if (!target.empty() && !isInPrefix(target, prefix))
+        abort();
+
+    nixDB.queryStrings(noTxn, dbId2Paths, id, paths);
+
+    /* Pick one equal to `target'. */
+    if (!target.empty()) {
+
+        for (Strings::iterator i = paths.begin();
+             i != paths.end(); i++)
+        {
+            string path = *i;
+            if (path == target && pathExists(path))
+                return path;
+        }
+        
+    }
+
+    /* Arbitrarily pick the first one that exists and isn't stale. */
+    for (Strings::iterator it = paths.begin();
+         it != paths.end(); it++)
+    {
+        string path = *it;
+        if (isInPrefix(path, prefix) && pathExists(path)) {
+            if (target.empty())
+                return path;
+            else {
+                /* Acquire a lock on the target path. */
+                Strings lockPaths;
+                lockPaths.push_back(target);
+                PathLocks outputLock(lockPaths);
+
+                /* Copy. */
+                copyPath(path, target);
+
+                /* Register the target path. */
+                Transaction txn(nixDB);
+                registerPath(txn, target, id);
+                txn.commit();
+
+                return target;
+            }
+        }
+    }
+
+    if (!ignoreSubstitutes) {
+        
+        if (pending.find(id) != pending.end())
+            throw Error(format("id %1% already being expanded") % (string) id);
+        pending.insert(id);
+
+        /* Try to realise the substitutes, but only if this id is not
+           already being realised by a substitute. */
+        Strings subs;
+        nixDB.queryStrings(noTxn, dbSubstitutes, id, subs); /* non-existence = ok */
+
+        for (Strings::iterator it = subs.begin(); it != subs.end(); it++) {
+            FSId subId = parseHash(*it);
+
+            debug(format("trying substitute %1%") % (string) subId);
+
+            realiseClosure(normaliseNixExpr(subId, pending), pending);
+
+            return expandId(id, target, prefix, pending);
+        }
+
+    }
+    
+    throw Error(format("cannot expand id `%1%'") % (string) id);
+}
+#endif
+
+    
+Path normaliseNixExpr(const Path & _nePath, PathSet pending)
 {
     Nest nest(lvlTalkative,
-        format("normalising nix expression %1%") % (string) id);
+        format("normalising expression in `%1%'") % (string) _nePath);
 
-    /* Try to substitute $id$ by any known successors in order to
-       speed up the rewrite process. */
-    id = useSuccessor(id);
+    /* Try to substitute the expression by any known successors in
+       order to speed up the rewrite process. */
+    Path nePath = useSuccessor(_nePath);
 
     /* Get the Nix expression. */
-    NixExpr ne = parseNixExpr(termFromId(id));
+    NixExpr ne = parseNixExpr(termFromPath(nePath));
 
     /* If this is a normal form (i.e., a closure) we are done. */
-    if (ne.type == NixExpr::neClosure) return id;
+    if (ne.type == NixExpr::neClosure) return nePath;
     if (ne.type != NixExpr::neDerivation) abort();
     
 
@@ -62,8 +146,8 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
     /* Input paths, with their closure elements. */
     ClosureElems inClosures; 
 
-    /* Referencable paths (i.e., input and output paths). */
-    StringSet allPaths;
+    /* Referenceable paths (i.e., input and output paths). */
+    PathSet allPaths;
 
     /* The environment to be passed to the builder. */
     Environment env; 
@@ -73,17 +157,17 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
     nf.type = NixExpr::neClosure;
 
 
-    /* Parse the outputs. */
-    for (DerivationOutputs::iterator i = ne.derivation.outputs.begin();
+    /* The outputs are referenceable paths. */
+    for (PathSet::iterator i = ne.derivation.outputs.begin();
          i != ne.derivation.outputs.end(); i++)
     {
-        debug(format("building %1% in `%2%'") % (string) i->second % i->first);
-        allPaths.insert(i->first);
+        debug(format("building path `%1%'") % *i);
+        allPaths.insert(*i);
     }
 
     /* Obtain locks on all output paths.  The locks are automatically
        released when we exit this function or Nix crashes. */
-    PathLocks outputLocks(pathsFromOutputs(ne.derivation.outputs));
+    PathLocks outputLocks(ne.derivation.outputs);
 
     /* Now check again whether there is a successor.  This is because
        another process may have started building in parallel.  After
@@ -94,13 +178,13 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
        other process can build this expression, so no further checks
        are necessary. */
     {
-        FSId id2 = useSuccessor(id);
-        if (id2 != id) {
-            NixExpr ne = parseNixExpr(termFromId(id2));
-            debug(format("skipping build of %1%, someone beat us to it")
-		  % (string) id);
+        Path nePath2 = useSuccessor(nePath);
+        if (nePath != nePath2) {
+            NixExpr ne = parseNixExpr(termFromPath(nePath2));
+            debug(format("skipping build of expression `%1%', someone beat us to it")
+		  % (string) nePath);
             if (ne.type != NixExpr::neClosure) abort();
-            return id2;
+            return nePath2;
         }
     }
 
@@ -110,14 +194,14 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
 		    % ne.derivation.platform % thisSystem);
         
     /* Realise inputs (and remember all input paths). */
-    for (FSIdSet::iterator i = ne.derivation.inputs.begin();
+    for (PathSet::iterator i = ne.derivation.inputs.begin();
          i != ne.derivation.inputs.end(); i++)
     {
-        FSId nf = normaliseNixExpr(*i, pending);
-        realiseClosure(nf, pending);
-        /* !!! nf should be a root of the garbage collector while we
-           are building */
-        NixExpr ne = parseNixExpr(termFromId(nf));
+        Path nfPath = normaliseNixExpr(*i, pending);
+        realiseClosure(nfPath, pending);
+        /* !!! nfPath should be a root of the garbage collector while
+           we are building */
+        NixExpr ne = parseNixExpr(termFromPath(nfPath));
         if (ne.type != NixExpr::neClosure) abort();
         for (ClosureElems::iterator j = ne.closure.elems.begin();
              j != ne.closure.elems.end(); j++)
@@ -147,8 +231,10 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
 
     /* We can skip running the builder if we can expand all output
        paths from their ids. */
+    bool fastBuild = false;
+#if 0
     bool fastBuild = true;
-    for (DerivationOutputs::iterator i = ne.derivation.outputs.begin();
+    for (PathSet::iterator i = ne.derivation.outputs.begin();
          i != ne.derivation.outputs.end(); i++)
     {
         try {
@@ -160,17 +246,17 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
             break;
         }
     }
+#endif
 
     if (!fastBuild) {
 
         /* If any of the outputs already exist but are not registered,
            delete them. */
-        for (DerivationOutputs::iterator i = ne.derivation.outputs.begin(); 
+        for (PathSet::iterator i = ne.derivation.outputs.begin(); 
              i != ne.derivation.outputs.end(); i++)
         {
-            string path = i->first;
-            FSId id;
-            if (queryPathId(path, id))
+            Path path = *i;
+            if (isValidPath(path))
                 throw Error(format("obstructed build: path `%1%' exists") % path);
             if (pathExists(path)) {
                 debug(format("removing unregistered path `%1%'") % path);
@@ -189,11 +275,11 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
     /* Check whether the output paths were created, and grep each
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
-    StringSet usedPaths;
-    for (DerivationOutputs::iterator i = ne.derivation.outputs.begin(); 
+    PathSet usedPaths;
+    for (PathSet::iterator i = ne.derivation.outputs.begin(); 
          i != ne.derivation.outputs.end(); i++)
     {
-        string path = i->first;
+        Path path = *i;
         if (!pathExists(path))
             throw Error(format("path `%1%' does not exist") % path);
         nf.closure.roots.insert(path);
@@ -207,15 +293,14 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
 
 	/* Construct a closure element for this output path. */
         ClosureElem elem;
-        elem.id = i->second;
 
 	/* For each path referenced by this output path, add its id to the
 	   closure element and add the id to the `usedPaths' set (so that the
 	   elements referenced by *its* closure are added below). */
-        for (Strings::iterator j = refPaths.begin();
+        for (Paths::iterator j = refPaths.begin();
 	     j != refPaths.end(); j++)
 	{
-	    string path = *j;
+	    Path path = *j;
 	    elem.refs.insert(path);
             if (inClosures.find(path) != inClosures.end())
                 usedPaths.insert(path);
@@ -228,11 +313,11 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
 
     /* Close the closure.  That is, for any referenced path, add the paths
        referenced by it. */
-    StringSet donePaths;
+    PathSet donePaths;
 
     while (!usedPaths.empty()) {
-	StringSet::iterator i = usedPaths.begin();
-	string path = *i;
+	PathSet::iterator i = usedPaths.begin();
+	Path path = *i;
 	usedPaths.erase(i);
 
 	if (donePaths.find(path) != donePaths.end()) continue;
@@ -243,7 +328,7 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
 
 	nf.closure.elems[path] = j->second;
 
-	for (StringSet::iterator k = j->second.refs.begin();
+	for (PathSet::iterator k = j->second.refs.begin();
 	     k != j->second.refs.end(); k++)
 	    usedPaths.insert(*k);
     }
@@ -252,7 +337,7 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
     for (ClosureElems::iterator i = inClosures.begin();
          i != inClosures.end(); i++)
     {
-        StringSet::iterator j = donePaths.find(i->first);
+        PathSet::iterator j = donePaths.find(i->first);
         if (j == donePaths.end())
             debug(format("NOT referenced: `%1%'") % i->first);
         else
@@ -263,7 +348,7 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
        transaction below because writing terms is idem-potent. */
     ATerm nfTerm = unparseNixExpr(nf);
     msg(lvlVomit, format("normal form: %1%") % printTerm(nfTerm));
-    FSId idNF = writeTerm(nfTerm, "-s-" + (string) id);
+    Path nfPath = writeTerm(nfTerm, "-s");
 
     /* Register each outpat path, and register the normal form.  This
        is wrapped in one database transaction to ensure that if we
@@ -272,63 +357,58 @@ FSId normaliseNixExpr(FSId id, FSIdSet pending)
        deleted arbitrarily, while registered paths can only be deleted
        by running the garbage collector. */
     Transaction txn(nixDB);
-    for (DerivationOutputs::iterator i = ne.derivation.outputs.begin(); 
+    for (PathSet::iterator i = ne.derivation.outputs.begin(); 
          i != ne.derivation.outputs.end(); i++)
-        registerPath(txn, i->first, i->second);
-    registerSuccessor(txn, id, idNF);
+        registerValidPath(txn, *i);
+    registerSuccessor(txn, nePath, nfPath);
     txn.commit();
 
-    return idNF;
+    return nfPath;
 }
 
 
-void realiseClosure(const FSId & id, FSIdSet pending)
+void realiseClosure(const Path & nePath, PathSet pending)
 {
-    Nest nest(lvlDebug, 
-        format("realising closure %1%") % (string) id);
+    Nest nest(lvlDebug, format("realising closure `%1%'") % nePath);
 
-    NixExpr ne = parseNixExpr(termFromId(id));
+    NixExpr ne = parseNixExpr(termFromPath(nePath));
     if (ne.type != NixExpr::neClosure)
-        throw Error(format("expected closure in %1%") % (string) id);
+        throw Error(format("expected closure in `%1%'") % nePath);
     
     for (ClosureElems::const_iterator i = ne.closure.elems.begin();
          i != ne.closure.elems.end(); i++)
+        assert(isValidPath(i->first));
+#if 0
         expandId(i->second.id, i->first, "/", pending);
+#endif
 }
 
 
-Strings nixExprPaths(const FSId & id)
+PathSet nixExprRoots(const Path & nePath)
 {
-    Strings paths;
+    PathSet paths;
 
-    NixExpr ne = parseNixExpr(termFromId(id));
+    NixExpr ne = parseNixExpr(termFromPath(nePath));
 
-    if (ne.type == NixExpr::neClosure) {
-        for (StringSet::const_iterator i = ne.closure.roots.begin();
-             i != ne.closure.roots.end(); i++)
-	    paths.push_back(*i);
-    }
-
-    else if (ne.type == NixExpr::neDerivation) {
-        for (DerivationOutputs::iterator i = ne.derivation.outputs.begin();
-             i != ne.derivation.outputs.end(); i++)
-            paths.push_back(i->first);
-    }
-    
+    if (ne.type == NixExpr::neClosure)
+        paths.insert(ne.closure.roots.begin(), ne.closure.roots.end());
+    else if (ne.type == NixExpr::neDerivation)
+        paths.insert(ne.derivation.outputs.begin(),
+            ne.derivation.outputs.end());
     else abort();
 
     return paths;
 }
 
 
-static void nixExprRequisitesSet(const FSId & id, 
-    bool includeExprs, bool includeSuccessors, StringSet & paths,
-    FSIdSet & doneSet)
+static void requisitesWorker(const Path & nePath,
+    bool includeExprs, bool includeSuccessors,
+    PathSet & paths, PathSet & doneSet)
 {
-    if (doneSet.find(id) != doneSet.end()) return;
-    doneSet.insert(id);
+    if (doneSet.find(nePath) != doneSet.end()) return;
+    doneSet.insert(nePath);
 
-    NixExpr ne = parseNixExpr(termFromId(id));
+    NixExpr ne = parseNixExpr(termFromPath(nePath));
 
     if (ne.type == NixExpr::neClosure)
         for (ClosureElems::iterator i = ne.closure.elems.begin();
@@ -336,35 +416,35 @@ static void nixExprRequisitesSet(const FSId & id,
             paths.insert(i->first);
     
     else if (ne.type == NixExpr::neDerivation)
-        for (FSIdSet::iterator i = ne.derivation.inputs.begin();
+        for (PathSet::iterator i = ne.derivation.inputs.begin();
              i != ne.derivation.inputs.end(); i++)
-            nixExprRequisitesSet(*i,
+            requisitesWorker(*i,
                 includeExprs, includeSuccessors, paths, doneSet);
 
     else abort();
 
-    if (includeExprs) 
-        paths.insert(expandId(id));
+    if (includeExprs) paths.insert(nePath);
 
-    string idSucc;
-    if (includeSuccessors &&
-        nixDB.queryString(noTxn, dbSuccessors, id, idSucc))
-        nixExprRequisitesSet(parseHash(idSucc), 
-            includeExprs, includeSuccessors, paths, doneSet);
+    string nfPath;
+    if (includeSuccessors && (nfPath = useSuccessor(nePath)) != nePath)
+        requisitesWorker(nfPath, includeExprs, includeSuccessors,
+            paths, doneSet);
 }
 
 
-Strings nixExprRequisites(const FSId & id,
+PathSet nixExprRequisites(const Path & nePath,
     bool includeExprs, bool includeSuccessors)
 {
-    StringSet paths;
-    FSIdSet doneSet;
-    nixExprRequisitesSet(id, includeExprs, includeSuccessors, paths, doneSet);
-    return Strings(paths.begin(), paths.end());
+    PathSet paths;
+    PathSet doneSet;
+    requisitesWorker(nePath, includeExprs, includeSuccessors,
+        paths, doneSet);
+    return paths;
 }
 
 
-FSIds findGenerators(const FSIds & _ids)
+#if 0
+PathSet findGenerators(const PathSet & outputs)
 {
     FSIdSet ids(_ids.begin(), _ids.end());
     FSIds generators;
@@ -407,3 +487,4 @@ FSIds findGenerators(const FSIds & _ids)
 
     return generators;
 }
+#endif
diff --git a/src/normalise.hh b/src/normalise.hh
index 9b8274681..4b4db4ee2 100644
--- a/src/normalise.hh
+++ b/src/normalise.hh
@@ -4,15 +4,22 @@
 #include "expr.hh"
 
 
-/* Normalise a Nix expression, that is, return an equivalent
-   closure.  (For the meaning of `pending', see expandId()). */
-FSId normaliseNixExpr(FSId id, FSIdSet pending = FSIdSet());
+/* Normalise a Nix expression.  That is, if the expression is a
+   derivation, a path containing an equivalent closure expression is
+   returned.  This requires that the derivation is performed, unless a
+   successor is known. */
+Path normaliseNixExpr(const Path & nePath, PathSet pending = PathSet());
 
-/* Realise a Closure in the file system. */
-void realiseClosure(const FSId & id, FSIdSet pending = FSIdSet());
+/* Realise a closure expression in the file system. 
+
+   The pending paths are those that are already being realised.  This
+   prevents infinite recursion for paths realised through a substitute
+   (since when we build the substitute, we would first try to realise
+   its output paths through substitutes... kaboom!). */
+void realiseClosure(const Path & nePath, PathSet pending = PathSet());
 
 /* Get the list of root (output) paths of the given Nix expression. */
-Strings nixExprPaths(const FSId & id);
+PathSet nixExprRoots(const Path & nePath);
 
 /* Get the list of paths that are required to realise the given
    expression.  For a derive expression, this is the union of
@@ -20,16 +27,16 @@ Strings nixExprPaths(const FSId & id);
    each element in the closure.  If `includeExprs' is true, include the
    paths of the Nix expressions themselves.  If `includeSuccessors' is
    true, include the requisites of successors. */
-Strings nixExprRequisites(const FSId & id,
+PathSet nixExprRequisites(const Path & nePath,
     bool includeExprs, bool includeSuccessors);
 
-/* Return the list of the ids of all known Nix expressions whose
-   output ids are completely contained in `ids'. */
-FSIds findGenerators(const FSIds & ids);
+/* Return the list of the paths of all known Nix expressions whose
+   output paths are completely contained in the set `outputs'. */
+PathSet findGenerators(const PathSet & outputs);
 
 /* Register a successor. */
 void registerSuccessor(const Transaction & txn,
-    const FSId & id1, const FSId & id2);
+    const Path & path1, const Path & path2);
 
 
 #endif /* !__NORMALISE_H */
diff --git a/src/pathlocks.cc b/src/pathlocks.cc
index 93f456ace..ff0226c84 100644
--- a/src/pathlocks.cc
+++ b/src/pathlocks.cc
@@ -13,20 +13,20 @@
 static StringSet lockedPaths; /* !!! not thread-safe */
 
 
-PathLocks::PathLocks(const Strings & _paths)
+PathLocks::PathLocks(const PathSet & _paths)
 {
     /* Note that `fds' is built incrementally so that the destructor
        will only release those locks that we have already acquired. */
 
     /* Sort the paths.  This assures that locks are always acquired in
        the same order, thus preventing deadlocks. */
-    Strings paths(_paths);
+    Paths paths(_paths.begin(), _paths.end());
     paths.sort();
     
     /* Acquire the lock for each path. */
-    for (Strings::iterator i = paths.begin(); i != paths.end(); i++) {
-        string path = *i;
-        string lockPath = path + ".lock";
+    for (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
+        Path path = *i;
+        Path lockPath = path + ".lock";
 
         debug(format("locking path `%1%'") % path);
 
@@ -64,6 +64,6 @@ PathLocks::~PathLocks()
     for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
         close(*i);
 
-    for (Strings::iterator i = paths.begin(); i != paths.end(); i++)
+    for (Paths::iterator i = paths.begin(); i != paths.end(); i++)
         lockedPaths.erase(*i);
 }
diff --git a/src/pathlocks.hh b/src/pathlocks.hh
index 03a62e65a..6c36b9b1d 100644
--- a/src/pathlocks.hh
+++ b/src/pathlocks.hh
@@ -8,10 +8,10 @@ class PathLocks
 {
 private:
     list<int> fds;
-    Strings paths;
+    Paths paths;
 
 public:
-    PathLocks(const Strings & _paths);
+    PathLocks(const PathSet & _paths);
     ~PathLocks();
 };
 
diff --git a/src/references.hh b/src/references.hh
index b19fbf72c..d009453d6 100644
--- a/src/references.hh
+++ b/src/references.hh
@@ -4,7 +4,7 @@
 #include "util.hh"
 
 
-Strings filterReferences(const string & path, const Strings & refs);
+Strings filterReferences(const Path & path, const Strings & refs);
 
 
 #endif /* !__VALUES_H */
diff --git a/src/shared.cc b/src/shared.cc
index c0f07e955..32e891617 100644
--- a/src/shared.cc
+++ b/src/shared.cc
@@ -72,5 +72,3 @@ int main(int argc, char * * argv)
 
     return 0;
 }
-
-
diff --git a/src/store.cc b/src/store.cc
index f05cdf3ba..b2b479e0d 100644
--- a/src/store.cc
+++ b/src/store.cc
@@ -8,7 +8,6 @@
 #include "db.hh"
 #include "archive.hh"
 #include "pathlocks.hh"
-#include "normalise.hh"
 
 
 struct CopySink : DumpSink
@@ -31,7 +30,7 @@ struct CopySource : RestoreSource
 };
 
 
-void copyPath(string src, string dst)
+void copyPath(const Path & src, const Path & dst)
 {
     debug(format("copying `%1%' to `%2%'") % src % dst);
 
@@ -82,7 +81,7 @@ void copyPath(string src, string dst)
 }
 
 
-void registerSubstitute(const FSId & srcId, const FSId & subId)
+void registerSubstitute(const Path & srcPath, const Path & subPath)
 {
 #if 0
     Strings subs;
@@ -98,202 +97,89 @@ void registerSubstitute(const FSId & srcId, const FSId & subId)
 
     /* For now, accept only one substitute per id. */
     Strings subs;
-    subs.push_back(subId);
+    subs.push_back(subPath);
     
     Transaction txn(nixDB);
-    nixDB.setStrings(txn, dbSubstitutes, srcId, subs);
+    nixDB.setStrings(txn, dbSubstitutes, srcPath, subs);
     txn.commit();
 }
 
 
-void registerPath(const Transaction & txn,
-    const string & _path, const FSId & id)
+void registerValidPath(const Transaction & txn, const Path & _path)
 {
-    string path(canonPath(_path));
-
-    debug(format("registering path `%1%' with id %2%")
-        % path % (string) id);
-
-    string oldId;
-    if (nixDB.queryString(txn, dbPath2Id, path, oldId)) {
-        if (id != parseHash(oldId))
-            throw Error(format("path `%1%' already contains id %2%")
-                % path % oldId);
-        return;
-    }
-
-    nixDB.setString(txn, dbPath2Id, path, id);
-
-    Strings paths;
-    nixDB.queryStrings(txn, dbId2Paths, id, paths); /* non-existence = ok */
-    
-    paths.push_back(path);
-    
-    nixDB.setStrings(txn, dbId2Paths, id, paths);
+    Path path(canonPath(_path));
+    debug(format("registering path `%1%'") % path);
+    nixDB.setString(txn, dbValidPaths, path, "");
 }
 
 
-void unregisterPath(const string & _path)
+bool isValidPath(const Path & path)
 {
-    string path(canonPath(_path));
+    string s;
+    return nixDB.queryString(noTxn, dbValidPaths, path, s);
+}
+
+
+void unregisterValidPath(const Path & _path)
+{
+    Path path(canonPath(_path));
     Transaction txn(nixDB);
 
     debug(format("unregistering path `%1%'") % path);
 
-    string _id;
-    if (!nixDB.queryString(txn, dbPath2Id, path, _id)) {
-        txn.abort();
-        return;
-    }
-    FSId id(parseHash(_id));
-
-    nixDB.delPair(txn, dbPath2Id, path);
-
-    Strings paths, paths2;
-    nixDB.queryStrings(txn, dbId2Paths, id, paths); /* non-existence = ok */
-
-    for (Strings::iterator it = paths.begin();
-         it != paths.end(); it++)
-        if (*it != path) paths2.push_back(*it);
-
-    nixDB.setStrings(txn, dbId2Paths, id, paths2);
+    nixDB.delPair(txn, dbValidPaths, path);
 
     txn.commit();
 }
 
 
-bool queryPathId(const string & path, FSId & id)
-{
-    string s;
-    if (!nixDB.queryString(noTxn, dbPath2Id, absPath(path), s)) return false;
-    id = parseHash(s);
-    return true;
-}
-
-
-bool isInPrefix(const string & path, const string & _prefix)
+static bool isInPrefix(const string & path, const string & _prefix)
 {
     string prefix = canonPath(_prefix + "/");
     return string(path, 0, prefix.size()) == prefix;
 }
 
 
-string expandId(const FSId & id, const string & target,
-    const string & prefix, FSIdSet pending, bool ignoreSubstitutes)
-{
-    Nest nest(lvlDebug, format("expanding %1%") % (string) id);
-
-    Strings paths;
-
-    if (!target.empty() && !isInPrefix(target, prefix))
-        abort();
-
-    nixDB.queryStrings(noTxn, dbId2Paths, id, paths);
-
-    /* Pick one equal to `target'. */
-    if (!target.empty()) {
-
-        for (Strings::iterator i = paths.begin();
-             i != paths.end(); i++)
-        {
-            string path = *i;
-            if (path == target && pathExists(path))
-                return path;
-        }
-        
-    }
-
-    /* Arbitrarily pick the first one that exists and isn't stale. */
-    for (Strings::iterator it = paths.begin();
-         it != paths.end(); it++)
-    {
-        string path = *it;
-        if (isInPrefix(path, prefix) && pathExists(path)) {
-            if (target.empty())
-                return path;
-            else {
-                /* Acquire a lock on the target path. */
-                Strings lockPaths;
-                lockPaths.push_back(target);
-                PathLocks outputLock(lockPaths);
-
-                /* Copy. */
-                copyPath(path, target);
-
-                /* Register the target path. */
-                Transaction txn(nixDB);
-                registerPath(txn, target, id);
-                txn.commit();
-
-                return target;
-            }
-        }
-    }
-
-    if (!ignoreSubstitutes) {
-        
-        if (pending.find(id) != pending.end())
-            throw Error(format("id %1% already being expanded") % (string) id);
-        pending.insert(id);
-
-        /* Try to realise the substitutes, but only if this id is not
-           already being realised by a substitute. */
-        Strings subs;
-        nixDB.queryStrings(noTxn, dbSubstitutes, id, subs); /* non-existence = ok */
-
-        for (Strings::iterator it = subs.begin(); it != subs.end(); it++) {
-            FSId subId = parseHash(*it);
-
-            debug(format("trying substitute %1%") % (string) subId);
-
-            realiseClosure(normaliseNixExpr(subId, pending), pending);
-
-            return expandId(id, target, prefix, pending);
-        }
-
-    }
-    
-    throw Error(format("cannot expand id `%1%'") % (string) id);
-}
-
-    
-void addToStore(string srcPath, string & dstPath, FSId & id,
-    bool deterministicName)
+Path addToStore(const Path & _srcPath)
 {
+    Path srcPath(absPath(_srcPath));
     debug(format("adding `%1%' to the store") % srcPath);
 
-    srcPath = absPath(srcPath);
-    id = hashPath(srcPath);
+    Hash h = hashPath(srcPath);
 
     string baseName = baseNameOf(srcPath);
-    dstPath = canonPath(nixStore + "/" + (string) id + "-" + baseName);
+    Path dstPath = canonPath(nixStore + "/" + (string) h + "-" + baseName);
 
-    try {
-        dstPath = expandId(id, deterministicName ? dstPath : "", 
-            nixStore, FSIdSet(), true);
-        return;
-    } catch (...) {
+    if (!isValidPath(dstPath)) { 
+
+        /* The first check above is an optimisation to prevent
+           unnecessary lock acquisition. */
+
+        PathSet lockPaths;
+        lockPaths.insert(dstPath);
+        PathLocks outputLock(lockPaths);
+
+        if (!isValidPath(dstPath)) {
+            copyPath(srcPath, dstPath);
+
+            Transaction txn(nixDB);
+            registerValidPath(txn, dstPath);
+            txn.commit();
+        }
     }
-    
-    Strings lockPaths;
-    lockPaths.push_back(dstPath);
-    PathLocks outputLock(lockPaths);
 
-    copyPath(srcPath, dstPath);
-
-    Transaction txn(nixDB);
-    registerPath(txn, dstPath, id);
-    txn.commit();
+    return dstPath;
 }
 
 
-void deleteFromStore(const string & path)
+void deleteFromStore(const Path & _path)
 {
-    string prefix =  + "/";
-    if (!isInPrefix(path, nixStore))
-        throw Error(format("path %1% is not in the store") % path);
+    Path path(canonPath(_path));
 
-    unregisterPath(path);
+    if (!isInPrefix(path, nixStore))
+        throw Error(format("path `%1%' is not in the store") % path);
+
+    unregisterValidPath(path);
 
     deletePath(path);
 }
@@ -305,6 +191,7 @@ void verifyStore()
 
     /* !!! verify that the result is consistent */
 
+#if 0
     Strings paths;
     nixDB.enumTable(txn, dbPath2Id, paths);
 
@@ -421,6 +308,7 @@ void verifyStore()
             }
         }
     }
+#endif
 
     txn.commit();
 }
diff --git a/src/store.hh b/src/store.hh
index 7f6b24569..69de82478 100644
--- a/src/store.hh
+++ b/src/store.hh
@@ -9,44 +9,27 @@
 using namespace std;
 
 
-typedef Hash FSId;
-
-typedef set<FSId> FSIdSet;
-
-
 /* Copy a path recursively. */
-void copyPath(string src, string dst);
+void copyPath(const Path & src, const Path & dst);
 
 /* Register a substitute. */
-void registerSubstitute(const FSId & srcId, const FSId & subId);
+void registerSubstitute(const Path & srcPath, const Path & subPath);
 
-/* Register a path keyed on its id. */
-void registerPath(const Transaction & txn,
-    const string & path, const FSId & id);
+/* Register the validity of a path. */
+void registerValidPath(const Transaction & txn, const Path & path);
 
-/* Query the id of a path. */
-bool queryPathId(const string & path, FSId & id);
+/* Unregister the validity of a path. */
+void unregisterValidPath(const Path & path);
 
-/* Return a path whose contents have the given hash.  If target is
-   not empty, ensure that such a path is realised in target (if
-   necessary by copying from another location).  If prefix is not
-   empty, only return a path that is an descendent of prefix.
+/* Checks whether a path is valid. */ 
+bool isValidPath(const Path & path);
 
-   The list of pending ids are those that already being expanded.
-   This prevents infinite recursion for ids realised through a
-   substitute (since when we build the substitute, we would first try
-   to expand the id... kaboom!). */
-string expandId(const FSId & id, const string & target = "",
-    const string & prefix = "/", FSIdSet pending = FSIdSet(),
-    bool ignoreSubstitutes = false);
-
-/* Copy a file to the nixStore directory and register it in dbRefs.
-   Return the hash code of the value. */
-void addToStore(string srcPath, string & dstPath, FSId & id,
-    bool deterministicName = false);
+/* Copy the contents of a path to the store and register the validity
+   the resulting path.  The resulting path is returned. */
+Path addToStore(const Path & srcPath);
 
 /* Delete a value from the nixStore directory. */
-void deleteFromStore(const string & path);
+void deleteFromStore(const Path & path);
 
 void verifyStore();
 
diff --git a/src/test.cc b/src/test.cc
index fb1e62eb3..457fecf24 100644
--- a/src/test.cc
+++ b/src/test.cc
@@ -10,10 +10,10 @@
 #include "globals.hh"
 
 
-void realise(FSId id)
+void realise(Path nePath)
 {
-    Nest nest(lvlDebug, format("TEST: realising %1%") % (string) id);
-    realiseClosure(normaliseNixExpr(id));
+    Nest nest(lvlDebug, format("TEST: realising `%1%'") % nePath);
+    realiseClosure(normaliseNixExpr(nePath));
 }
 
 
@@ -48,12 +48,12 @@ void runTests()
     try {
         h = parseHash("blah blah");
         abort();
-    } catch (BadRefError err) { };
+    } catch (Error err) { };
 
     try {
         h = parseHash("0b0ffd0538622bfe20b92c4aa57254d99");
         abort();
-    } catch (BadRefError err) { };
+    } catch (Error err) { };
 
     /* Path canonicalisation. */
     cout << canonPath("/./../././//") << endl;
@@ -97,76 +97,59 @@ void runTests()
 
     /* Expression evaluation. */
 
-    FSId builder1id;
-    string builder1fn;
-    addToStore("./test-builder-1.sh", builder1fn, builder1id);
+    Path builder1fn;
+    builder1fn = addToStore("./test-builder-1.sh");
 
     ATerm fs1 = ATmake(
-        "Closure([<str>], [(<str>, <str>, [])])",
+        "Closure([<str>], [(<str>, [])])",
         builder1fn.c_str(),
-        builder1fn.c_str(),
-        ((string) builder1id).c_str());
-    FSId fs1id = writeTerm(fs1, "");
+        builder1fn.c_str());
+    Path fs1ne = writeTerm(fs1, "-c");
 
-    realise(fs1id);
-    realise(fs1id);
+    realise(fs1ne);
+    realise(fs1ne);
 
-    ATerm fs2 = ATmake(
-        "Closure([<str>], [(<str>, <str>, [])])",
-        (builder1fn + "_bla").c_str(),
-        (builder1fn + "_bla").c_str(),
-        ((string) builder1id).c_str());
-    FSId fs2id = writeTerm(fs2, "");
-
-    realise(fs2id);
-    realise(fs2id);
-
-    string out1id = hashString("foo"); /* !!! bad */
-    string out1fn = nixStore + "/" + (string) out1id + "-hello.txt";
+    string out1h = hashString("foo"); /* !!! bad */
+    Path out1fn = nixStore + "/" + (string) out1h + "-hello.txt";
     ATerm fs3 = ATmake(
-        "Derive([(<str>, <str>)], [<str>], <str>, <str>, [], [(\"out\", <str>)])",
+        "Derive([<str>], [<str>], <str>, <str>, [], [(\"out\", <str>)])",
         out1fn.c_str(),
-        ((string) out1id).c_str(),
-        ((string) fs1id).c_str(),
+        fs1ne.c_str(),
         thisSystem.c_str(),
-        ((string) builder1fn).c_str(),
+        builder1fn.c_str(),
         out1fn.c_str());
     debug(printTerm(fs3));
-    FSId fs3id = writeTerm(fs3, "");
+    Path fs3ne = writeTerm(fs3, "-d");
 
-    realise(fs3id);
-    realise(fs3id);
+    realise(fs3ne);
+    realise(fs3ne);
 
 
-    FSId builder4id;
-    string builder4fn;
-    addToStore("./test-builder-2.sh", builder4fn, builder4id);
+    Path builder4fn = addToStore("./test-builder-2.sh");
 
     ATerm fs4 = ATmake(
-        "Closure([<str>], [(<str>, <str>, [])])",
+        "Closure([<str>], [(<str>, [])])",
         builder4fn.c_str(),
-        builder4fn.c_str(),
-        ((string) builder4id).c_str());
-    FSId fs4id = writeTerm(fs4, "");
+        builder4fn.c_str());
+    Path fs4ne = writeTerm(fs4, "-c");
 
-    realise(fs4id);
+    realise(fs4ne);
 
-    string out5id = hashString("bar"); /* !!! bad */
-    string out5fn = nixStore + "/" + (string) out5id + "-hello2";
+    string out5h = hashString("bar"); /* !!! bad */
+    Path out5fn = nixStore + "/" + (string) out5h + "-hello2";
     ATerm fs5 = ATmake(
-        "Derive([(<str>, <str>)], [<str>], <str>, <str>, [], [(\"out\", <str>), (\"builder\", <str>)])",
+        "Derive([<str>], [<str>], <str>, <str>, [], [(\"out\", <str>), (\"builder\", <str>)])",
         out5fn.c_str(),
-        ((string) out5id).c_str(),
-        ((string) fs4id).c_str(),
+        fs4ne.c_str(),
         thisSystem.c_str(),
-        ((string) builder4fn).c_str(),
+        builder4fn.c_str(),
         out5fn.c_str(),
-        ((string) builder4fn).c_str());
+        builder4fn.c_str());
     debug(printTerm(fs5));
-    FSId fs5id = writeTerm(fs5, "");
+    Path fs5ne = writeTerm(fs5, "-d");
 
-    realise(fs5id);
-    realise(fs5id);
+    realise(fs5ne);
+    realise(fs5ne);
 }