/**
* Returns a substring of a string, starting at a specified index position
* and including the specified number of characters. The index is zero-based.
*
* The arguments can be any valid expression as long as long as the first argument resolves to a string, and the second and third arguments resolve to integers.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/substr/
* @see Expr::substr
* @param mixed|Expr $string
* @param mixed|Expr $start
* @param mixed|Expr $length
* @return self
*/
public function substr($string, $start, $length)
{
$this->expr->substr($string, $start, $length);
return $this;
}
