def unset(db, collection, field, match=None):
c = db[collection]<EOL>match = match if match is not None else {}<EOL>if db.client.server_info()['<STR_LIT:version>'].startswith('<STR_LIT:2>'):<EOL><INDENT>c.update(match, {'<STR_LIT>': {field: '<STR_LIT>'}}, multi=True)<EOL><DEDENT>else:<EOL><INDENT>c.update_many(match, {'<STR_LIT>': {field: '<STR_LIT>'}})<EOL><DEDENT>
Removes ``field`` from all records in ``collection`` that meet
``match`` criteria.

Arguments:

    field (str): Field to be removed.

    db (Database): A pymongo Database object.

    collection (str): Collection name.

    match (dict): A dictionary containing the match criteria, for example::

        {'seq_id': {'$in': ['a', 'b', 'c']}, 'cdr3_len': {'$gte': 18}}