private UpgradePack parseServiceUpgradePack(UpgradePack parent, UpgradePack child) {
    parent.mergePrerequisiteChecks(child);
    parent.mergeProcessing(child);

    return child;
  }
Places prerequisite checks and processing objects onto the parent upgrade pack.

@param parent  the parent upgrade pack
@param child   the parsed child upgrade pack
@return the child upgrade pack
ambari-server/src/main/java/org/apache/ambari/server/stack/StackModule.java