def LET(_in, _vars):
    """
    Binds variables for use in the specified expression, and returns the result of the expression.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/let/
    for more details
    :param _in: The expression to evaluate.
    :param _vars: Assignment block for the variables accessible in the in expression.
    :return: Aggregation operator
    """
    return {'$let': {'vars': _vars, 'in': _in}}
