/**
* @param mixed|null $payload
* @return mixed|null
*/
public function process($payload = null)
{
if (empty($this->stages)) {
return null;
}
// Call all stages which depend only on the input
$graph = new Graph($this->stages, new Sequential());
$results = $graph->process($payload);
end($this->stages);
$lastStageIndex = key($this->stages);
reset($this->stages);
return $results->output($lastStageIndex);
}
