docs: def get_chain_mapping(self, pdb_name1, pdb_name2):
          '''This replaces the old mapping member by constructing it from self.chain_mapping. This function returns a mapping from
          chain IDs in pdb_name1 to chain IDs in pdb_name2.'''
          raise Exception('Implement. Map pdb_namex to its equivalence class, call get_representative_chain_mapping, and something something.')
          pprint.pprint(self.chain_mapping)
  
          d = {}
          for pdb1_chain_id, matched_chain_list in self.chain_mapping[(pdb_name1, pdb_name2)].iteritems():
              d[pdb1_chain_id] = matched_chain_list.get_related_chains_ids(pdb_name2)
          return d