SumBag: Implement membership tests

This is not strictly required by copar but allows for easier testing.

src/Data/SumBag.hs

@@ -7,9 +7,10 @@ module Data.SumBag
   , sum
   , insert
   , delete
+  , elem
   ) where
 
-import Prelude hiding (sum, min)
+import Prelude hiding (sum, min, elem)
 import qualified Data.List.NonEmpty as NE
 
 type SumBag a = Tree a
@@ -57,6 +58,13 @@ insert a (Node _ e left right)
   | a > value e = balance1 e left (insert a right)
   | otherwise = node (addOnce e) left right
 
+elem :: (Ord a) => a -> SumBag a -> Bool
+elem _ Leaf = False
+elem a (Node _ e left right)
+  | a < value e = elem a left
+  | a > value e = elem a right
+  | otherwise = True
+
 delete :: (Ord a, Monoid a) => a -> SumBag a -> SumBag a
 delete _ Leaf = Leaf
 delete a (Node _ e left right)