code: def dependency_list(self, tkn: str) -> List[str]:
          
          if tkn not in self.dependency_map:
              self.dependency_map[tkn] = [tkn]        # Force a circular reference
              self.dependency_map[tkn] = self.reference(tkn).dependency_list()
          return self.dependency_map[tkn]
docs: Return a list all of the grammarelts that depend on tkn
  
          :param tkn: 
          :return: