docs: document set theory symbols

These symbols confused me so I wanted to gloss them to help future users. You can see the context here: https://discourse.nixos.org/t/flakes-what-does-mean-big-bundle-of-questions
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Eli Flanagan 2021-11-30 11:13:46 -05:00
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the store object at `P` contains the path `Q` somewhere. The the store object at `P` contains the path `Q` somewhere. The
*references* of a store path are the set of store paths to which it *references* of a store path are the set of store paths to which it
has a reference. has a reference.
A derivation can reference other derivations and sources (but not A derivation can reference other derivations and sources (but not
output paths), whereas an output path only references other output output paths), whereas an output path only references other output
paths. paths.
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is necessary to deploy whole closures, since otherwise at runtime is necessary to deploy whole closures, since otherwise at runtime
files could be missing. The command `nix-store -qR` prints out files could be missing. The command `nix-store -qR` prints out
closures of store paths. closures of store paths.
As an example, if the store object at path `P` contains a reference As an example, if the store object at path `P` contains a reference
to path `Q`, then `Q` is in the closure of `P`. Further, if `Q` to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
references `R` then `R` is also in the closure of `P`. references `R` then `R` is also in the closure of `P`.
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store. It can contain regular files, directories and symbolic store. It can contain regular files, directories and symbolic
links. NARs are generated and unpacked using `nix-store --dump` links. NARs are generated and unpacked using `nix-store --dump`
and `nix-store --restore`. and `nix-store --restore`.
- `∅` \
The empty set symbol. In the context of package history, this denotes a package was not present prior to installation.
- `ε` \
The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.