// returns previous mounted
static O mountObjectOnObject(O mounter, O mounted, O key) {
  if (mounted == null) null;
  Map generalMap = vm_generalMap();
  if (generalMap == null) fail("Please upgrade JavaX");
  synchronized(generalMap) {
    Map map = cast generalMap.get("mounts");
    if (map == null) {
      pcall-short {
        map = (Map) nuForeignObject(javax(), className(javax()) + '$WeakIdentityHashMap);
      }
      if (map == null) map = new WeakIdentityHashMap; // older JavaX
      generalMap.put("mounts", map);
    }
    Map m = cast map.get(mounted);
    if (m == null)
      map.put(mounted, m = new HashMap);
      
    ret m.put(key, mounter);
  }
}

Began life as a copy of #1010611