diff --git a/src/SES.hs b/src/SES.hs
index 991dfe9bf..1a5277fec 100644
--- a/src/SES.hs
+++ b/src/SES.hs
@@ -10,13 +10,18 @@ import Data.List (uncons)
 import qualified Data.Map as Map
 import Data.Ord (comparing)
 
+-- | A function that maybe creates a diff from two terms.
 type Compare a annotation = Term a annotation -> Term a annotation -> Maybe (Diff a annotation)
+
+-- | A function that computes the cost of a diff.
 type Cost a annotation = Diff a annotation -> Integer
 
+-- | Find the shortest edit script (diff) between two terms given a function to compute the cost.
 ses :: Compare a annotation -> Cost a annotation -> [Term a annotation] -> [Term a annotation] -> [Diff a annotation]
 ses diffTerms cost as bs = fst <$> evalState diffState Map.empty where
   diffState = diffAt diffTerms cost (0, 0) as bs
 
+-- | Find the shortest edit script between two terms at a given vertex in the edit graph.
 diffAt :: Compare a annotation -> Cost a annotation -> (Integer, Integer) -> [Term a annotation] -> [Term a annotation] -> State (Map.Map (Integer, Integer) [(Diff a annotation, Integer)]) [(Diff a annotation, Integer)]
 diffAt _ _ _ [] [] = return []
 diffAt _ cost _ [] bs = return $ foldr toInsertions [] bs where
@@ -46,5 +51,6 @@ diffAt diffTerms cost (i, j) (a : as) (b : bs) = do
     best = minimumBy (comparing costOf)
     recur = diffAt diffTerms cost
 
+-- | Prepend a diff to the list with the cumulative cost.
 consWithCost :: Cost a annotation -> Diff a annotation -> [(Diff a annotation, Integer)] -> [(Diff a annotation, Integer)]
 consWithCost cost diff rest = (diff, cost diff + maybe 0 snd (fst <$> uncons rest)) : rest