public function setDifference($expression1, $expression2) : self
    {
        $this->expr->setDifference($expression1, $expression2);

        return $this;
    }
public function setDifference($expression1, $expression2) : self
    {
        $this->expr->setDifference($expression1, $expression2);

        return $this;
    }
public function setDifference ( $ expression1 , $ expression2 ) : self { $ this -> expr -> setDifference ( $ expression1 , $ expression2 ) ; return $ this ; }
Takes two sets and returns an array containing the elements that only
exist in the first set.

The arguments can be any valid expression as long as they each resolve to an array.

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

@param mixed|Expr $expression1
@param mixed|Expr $expression2
Takes two sets and returns an array containing the elements that only exist in the first set .