docs: def get_sequences(self, pdb_id = None):
          '''Create Sequence objects for each FASTA sequence.'''
          sequences = {}
          if pdb_id:
              for chain_id, sequence in self.get(pdb_id, {}).iteritems():
                  sequences[chain_id] = Sequence.from_sequence(chain_id, sequence)
          else:
              for pdb_id, v in self.iteritems():
                  sequences[pdb_id] = {}
                  for chain_id, sequence in v.iteritems():
                      sequences[pdb_id][chain_id] = Sequence.from_sequence(chain_id, sequence)
          return sequences