/**
* Returns an array of all unique values that results from applying an
* expression to each document in a group of documents that share the same
* group by key. Order of the elements in the output array is unspecified.
*
* AddToSet is an accumulator operation only available in the group stage.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/addToSet/
* @see Expr::addToSet
* @param mixed|Expr $expression
* @return Operator
*/
public function addToSet($expression)
{
$this->expr->addToSet($expression);
return $this;
}