<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def head_ref(self):
 """ Filters the current DataFrame to only contain those rows whose reference is HEAD. :rtype: ReferencesDataFrame """

        return ReferencesDataFrame(self._engine_dataframe.getHEAD(),
                                   self._session, self._implicits)