diff --git a/polymani.pl b/polymani.pl
index db44eca..de9a2ca 100644
--- a/polymani.pl
+++ b/polymani.pl
@@ -680,7 +680,7 @@ parse_polynomial_explicit(TLP-TL, T) -->
     %% on the left, through the difference structure,
     parse_polynomial_operand(TL),
     %% parse either a minus or a plus
-    member(COp, [-, +]),
+    { member(COp, [-, +]) },
     parse_operation(COp),
     !,
     %% Recurse on the right with add; position the sub tree on the right