def normalize(records):
    """
    Normalize result by removing the zone_time_first and zone_time_last keys
    and adding a source [sensor or zone] key pair.

    :param records: List (of dictionaries)
    :return: List (of dictionaries)
    """
    normalized = []
    for record in records:
        normalized_record = dict()
        normalized_record['source'] = 'sensor'
        keys = record.keys()
        for key in keys:
            if key == 'zone_time_first':
                normalized_record['time_first'] = record[key]
                normalized_record['source'] = 'zone'
            elif key == 'zone_time_last':
                normalized_record['time_last'] = record[key]
                normalized_record['source'] = 'zone'
            else:
                normalized_record[key] = record[key]
        normalized.append(normalized_record)
    return normalized
