diff --git a/ixset-typed.cabal b/ixset-typed.cabal
--- a/ixset-typed.cabal
+++ b/ixset-typed.cabal
@@ -1,5 +1,5 @@
 name:                ixset-typed
-version:             0.1.2
+version:             0.1.3
 synopsis:            Efficient relational queries on Haskell sets.
 description:
     Create and query sets that are indexed by multiple indices.
diff --git a/src/Data/IxSet/Typed.hs b/src/Data/IxSet/Typed.hs
--- a/src/Data/IxSet/Typed.hs
+++ b/src/Data/IxSet/Typed.hs
@@ -108,6 +108,7 @@
      -- * Set type
      IxSet(),
      Indexable(..),
+     IsIndexOf(),
      All,
      -- ** Declaring indices
      Ix(),
@@ -261,6 +262,8 @@
 zipWithIxList f (x ::: xs) (y ::: ys) = f x y ::: zipWithIxList f xs ys
 zipWithIxList _ _          _          = error "Data.IxSet.Typed.zipWithIxList: impossible"
 
+-- Constraint for membership in the type-level list. Says that 'ix'
+-- is contained in the index list 'ixs'.
 class Ord ix => IsIndexOf (ix :: *) (ixs :: [*]) where
   access :: IxList ixs a -> Ix ix a
   mapAt :: (All Ord ixs)