@staticmethod
def _get_crossreference(dct, rel, dst):
    """Get pathway crossReference."""
    assert dst['databaseName'] in set(['BioModels Database', 'OMIM']), dst
    assert dst['schemaClass'] == 'DatabaseIdentifier'
    assert rel['stoichiometry'] == 1
    if 'crossreference' not in dct:
        dct['crossreference'] = [dst['displayName']]
    else:
        dct['crossreference'].append(dst['displayName'])
