/**
* Evaluates a boolean expression to return one of the two specified return
* expressions.
*
* The arguments can be any valid expression.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/cond/
* @see Expr::cond
* @param mixed|Expr $if
* @param mixed|Expr $then
* @param mixed|Expr $else
* @return self
*/
public function cond($if, $then, $else)
{
$this->expr->cond($if, $then, $else);
return $this;
}