public function let($vars, $in) : self
    {
        $this->expr->let($vars, $in);

        return $this;
    }
public function let($vars, $in) : self
    {
        $this->expr->let($vars, $in);

        return $this;
    }
public function let ( $ vars , $ in ) : self { $ this -> expr -> let ( $ vars , $ in ) ; return $ this ; }
Binds variables for use in the specified expression, and returns the
result of the expression.

@see http://docs.mongodb.org/manual/reference/operator/aggregation/let/
@see Expr::let

@param mixed|Expr $vars Assignment block for the variables accessible in the in expression. To assign a variable, specify a string for the variable name and assign a valid expression for the value.
@param mixed|Expr $in   The expression to evaluate.
Binds variables for use in the specified expression and returns the result of the expression .