/**
* Retrieves the sub-milestones of a given parent milestone artifact.
*
* @param User $user
* @param Tracker_Artifact $milestone_artifact
*
* @return array of Tracker_Artifact
*/
private function getSubMilestonesArtifacts(User $user, Tracker_Artifact $milestone_artifact)
{
return array_values($milestone_artifact->getHierarchyLinkedArtifacts($user));
}
