code: public function returnsByReferenceAt($timing, $method, &$reference, $args = false)
      {
          $this->dieOnNoMethod($method, 'set return reference sequence');
          $this->actions->registerAt($timing, $method, $args, new SimpleByReference($reference));
      }
docs: Sets a return for a parameter list that
  will be passed by value only when the required call count is reached.
  
  @param int $timing        Number of calls in the future to which the result applies.
  If not set then all calls will return the value.
  @param string $method     Method name.
  @param mixed $reference   Result of the call will be this object.
  @param array $args        List of parameters to match including wildcards.