diff --git a/site.hs b/site.hs
index 985ce23..1a17d58 100644
--- a/site.hs
+++ b/site.hs
@@ -233,6 +233,8 @@ sourceTags = do
     use pages
   htags .= M.fromList (invTags sgat)
 
+-- | Organize a list of pages with hierarchical tags to a list with
+-- hierarchical tags with pages attached.
 invTags :: [(FilePath, [[String]])] -> [([String], [FilePath])]
 invTags x =
   map (fmap nub) $ groupSort [(t, p) | (p, htl) <- x, ht <- htl, t <- inits ht]