<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_raw_code(self, string_or_list):
 """Add raw Gmsh code. """

        if _is_string(string_or_list):
            self._GMSH_CODE.append(string_or_list)
        else:
            assert isinstance(string_or_list, list)
            for string in string_or_list:
                self._GMSH_CODE.append(string)
        return