def SUM(*expression):
    """
    Calculates and returns the sum of numeric values.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/sum/
    for more details
    :param expression: expression or variables
    :return: Aggregation operator
    """
    return {'$sum': list(expression)} if len(expression) > 1 else {'$sum':
        expression[0]} if expression else {}
