diff --git a/Library/Homebrew/cleaner.rb b/Library/Homebrew/cleaner.rb
index 30cf1c0f5d..e80cdfe92e 100644
--- a/Library/Homebrew/cleaner.rb
+++ b/Library/Homebrew/cleaner.rb
@@ -67,6 +67,12 @@ class Cleaner
   # Clean a top-level (bin, sbin, lib) directory, recursively, by fixing file
   # permissions and removing .la files, unless the files (or parent
   # directories) are protected by skip_clean.
+  #
+  # bin and sbin should not have any subdirectories; if either do that is
+  # caught as an audit warning
+  #
+  # lib may have a large directory tree (see Erlang for instance), and
+  # clean_dir applies cleaning rules to the entire tree
   def clean_dir d
     d.find do |path|
       path.extend(ObserverPathnameExtension)