fix remove_extra_brackets undoing work of post-processing step

inc/expression.hpp

@@ -277,6 +277,21 @@ public:
 			return "Invalid operator";
 		}
 	}
+
+	int negative_operator(void) const
+	{
+		if (t != OPERATOR)
+		{
+			return 0;
+		}
+
+		return (v.o == SUB || v.o == DIV);
+	}
+
+	static int is_negative(Operator o)
+	{
+		return (o == SUB || o == DIV);
+	}
 };
 
 struct Operation