Improved isEqual semantics

Eval.hs

@@ -26,8 +26,9 @@ eval env expr = let (eval', evalFull') = (eval env, evalFull env) in case expr o
                 ([Set records], [lambda]) -> Set (map (\record -> eval' $ FuncCall lambda [] [record]) records)
                 _ -> error "Map argument error"
 
-            IsEqual -> let (e1:e2:_) = args' in -- TODO not sufficent.
-                Boolean (e1 == e2)
+            IsEqual -> case args' of -- TODO not sufficent.
+                [e1, e2] -> Boolean (e1 == e2)
+                _ -> "isEqual argument error"
             
             XProduct -> case inputSets' of
                 [(Set l1), (Set l2)] -> Set $ [ x `concatRecord` y | x <- l1, y <- l2]