diff --git a/src/lib/Hydra/Helper/BuildDiff.pm b/src/lib/Hydra/Helper/BuildDiff.pm
index 6dc534df..ad5cf59b 100644
--- a/src/lib/Hydra/Helper/BuildDiff.pm
+++ b/src/lib/Hydra/Helper/BuildDiff.pm
@@ -22,6 +22,7 @@ sub cmpBuilds {
 
 sub buildDiff {
     my ($builds, $builds2) = @_;
+    # $builds is the list of current builds, and $builds2 is the list of previous (compared-to) builds
 
     my $ret = {
         stillSucceed => [],