public function appendElementToMilestoneBacklog($linked_artifact_id, PFUser $current_user, Planning_Milestone $milestone)
 {
     $linked_artifact_ids = $this->artifactlink_updater->getElementsAlreadyLinkedToArtifact($milestone->getArtifact(), $current_user);
     array_push($linked_artifact_ids, $linked_artifact_id);
     $this->updateMilestoneContent(array_unique($linked_artifact_ids), $current_user, $milestone);
 }