static O collectionMutex(O o) {
  S c = className(o);
  if (eq(c, "java.util.TreeMap$KeySet"))
    c = className(o = get(o, 'm));

  ifdef collectionMutex_debug
    print("Class name: " + className(o));
  endifdef
  
  if (eq(c, "java.util.TreeMap$AscendingSubMap"))
    c = className(o = get(o, 'm));
  
  ret o;
}