/**
* Compares two values and returns:
* true when the first value is less than the second value.
* false when the first value is greater than or equivalent to the second value.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/lt/
* @see Expr::lt
* @param mixed|Expr $expression1
* @param mixed|Expr $expression2
* @return self
*/
public function lt($expression1, $expression2)
{
$this->expr->lt($expression1, $expression2);
return $this;
}