code: def ref(self, ref):
          
          return ReferencesDataFrame(self.filter(self.name == ref)._jdf,
                                     self._session, self._implicits)
docs: Filters the current DataFrame to only contain those rows whose reference is the given
          reference name.
  
          >>> heads_df = refs_df.ref('refs/heads/HEAD')
  
          :param ref: Reference to get
          :type ref: str
          :rtype: ReferencesDataFrame