Make the precondition for FCo preservation stricter

FCO/Test.hs

@@ -18,7 +18,7 @@ args = Args{replay=Nothing, maxSuccess=1000, maxDiscardRatio=1000000, maxSize=10
 
 -- The type of a term does not change when a reduction step is performed
 coPreservation :: Term -> Property
-coPreservation te = (isRight ty) && (not $ CoI.isVal te)
+coPreservation te = (isRight ty) && (not $ CoI.isVal te) && (isJust $ CoI.stepEval te)
                 ==> ty == tyStep
   where ty = CoI.typeOf te CoI.Nil
         tyStep = (flip CoI.typeOf) CoI.Nil $ fromMaybe undefined $ CoI.stepEval te