def SLICE(array, n, position=None):
    """
    Returns a subset of an array.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/slice/
    for more details
    :param array: Any valid expression as long as it resolves to an array.
    :param n: Any valid expression as long as it resolves to an integer.
    :param position: Optional. Any valid expression as long as it resolves to an integer.
    :return: Aggregation operator
    """
    return {'$slice': [array, position, n]} if position is not None else {'$slice': [array, n]}