fix missing stack

Author: phtournier

freefem.py

@@ -984,8 +984,8 @@ class FreeFemLexer(CppLexer):
 		'using'
     ))
 
-    def get_tokens_unprocessed(self, text):
-        for index, token, value in CppLexer.get_tokens_unprocessed(self, text):
+    def get_tokens_unprocessed(self, text, stack=('root',)):
+        for index, token, value in CppLexer.get_tokens_unprocessed(self, text, stack):
             if value in self.operators:
                 yield index, Operator, value
             elif value in self.types: