public function __construct($id, $sourceId = null) {
if (null === $sourceId) {
$msg = sprintf('You have requested a non-existent service "%s".', $id);
}
else {
$msg = sprintf('The service "%s" has a dependency on a non-existent service "%s".', $sourceId, $id);
}
parent::__construct($msg);
$this->id = $id;
$this->sourceId = $sourceId;
}