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