def unset(db, collection, field, match=None):
    """
    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}}
    """
    c = db[collection]
    match = match if match is not None else {}
    if db.client.server_info()['version'].startswith('2'):
        c.update(match, {'$unset': {field: ''}}, multi=True)
    else:
        c.update_many(match, {'$unset': {field: ''}})
