A helper function to return the Sequence for a chain. If use_seqres_sequences_if_possible then we return the SEQRES
           Sequence if it exists. We return a tuple of values, the first identifying which sequence was returned.

def get_annotated_chain_sequence_string(self, chain_id, use_seqres_sequences_if_possible, raise_Exception_if_not_found = True):
        '''A helper function to return the Sequence for a chain. If use_seqres_sequences_if_possible then we return the SEQRES
           Sequence if it exists. We return a tuple of values, the first identifying which sequence was returned.'''
        if use_seqres_sequences_if_possible and self.seqres_sequences and self.seqres_sequences.get(chain_id):
            return ('SEQRES', self.seqres_sequences[chain_id])
        elif self.atom_sequences.get(chain_id):
            return ('ATOM', self.atom_sequences[chain_id])
        elif raise_Exception_if_not_found:
            raise Exception('Error: Chain %s expected but not found.' % (str(chain_id)))
        else:
            return None