Add paren expression

Signed-off-by: CHatingPython <chatingpython@gmail.com>

furc/src/main.cpp

@@ -5,7 +5,7 @@
 #include <iostream>
 
 int main(void) {
-    furc::front::parser parser("<TEMP>", "func main() {\n    return 6 - -7 * 9++;\n}");
+    furc::front::parser parser("<TEMP>", "func main() {\n    return (6 - -7) * 9++;\n}");
     std::cout << parser.parse() << '\n';
 
     return 0;