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