[semantically fix Data.MySet.deleteM
Helmut Grohne <grohne@cs.uni-bonn.de>**20150113142231
 Ignore-this: 449659c021ec64dbdc96c2ca21e21d83
 
 Previously deleteM would either return Just empty or Nothing. Likely, the
 intention was to either return Just a non-empty set or Nothing, but it makes no
 practical difference on known examples.
] hunk ./src/Data/MySet.hs 18
-    liftM (delete r) rs >>=  \rs' -> 
-        if null rs' then return rs' else Nothing
+    liftM (delete r) rs >>=  \rs' ->
+        if null rs' then Nothing else return rs'