diff --git a/src/Data/RandomWalkSimilarity.hs b/src/Data/RandomWalkSimilarity.hs
index ffed24357..2a28a6875 100644
--- a/src/Data/RandomWalkSimilarity.hs
+++ b/src/Data/RandomWalkSimilarity.hs
@@ -44,6 +44,9 @@ rws compare getLabel as bs
                 pure [ (i, compared) ]
         deleteRemaining diffs unmapped = foldl' (flip (List.insertBy (comparing fst))) (join diffs) ((,) (negate 1) . delete . snd . snd <$> unmapped)
 
+data UnmappedTerm leaf annotation = UnmappedTerm { index :: {-# UNPACK #-} !Int, feature :: !(Vector.Vector Double), term :: !(Term leaf annotation) }
+  deriving Eq
+
 longestIncreasingSubsequence :: Ord a => [a] -> [a]
 longestIncreasingSubsequence = longestOrderedSubsequenceBy (<)