docs: def sequence(pdb_filepath):
      '''A convenience method for printing PDB sequences in command-line execution.
  
          :param pdb_filepath: A path to a PDB file.
          :return: A string where odd line is a chain letter and each following even line is that chain's ATOM sequence.
  
          Example use:
              from klab.bio import pdb
              print(pdb.sequence('1234.pdb'))
      '''
      return '\n'.join(['{0}\n{1}'.format(chain_id, str(seq)) for chain_id, seq in sorted(PDB.from_filepath(pdb_filepath).atom_sequences.iteritems())])