def SUB_STR(string, start, length):
    """
    Returns a substring of a string, starting at a specified index position
    and including the specified number of characters.
    https://docs.mongodb.com/manual/reference/operator/aggregation/substr/
    for more details
    :param string: The string or expression of string
    :param start: Indicates the starting point of the substring.
    :param length: Can be any valid expression as long as it resolves to a non-negative integer or number
    :return: Aggregation operator
    """
    return {'$substr': [string, start, length]}
