static new HashSet<S> fillVar_set;

// make: func -> var type; or a class
// TODO: better synchronisation
// also, might return null while var is filled by another thread
static O fillVar(S var, O make) {
  synchronized(fillVar_set) {
    if (fillVar_set.contains(var)) ret get(mc(), var);
    fillVar_set.add(var);
  }
  O o = make instanceof Class ? nuInstance((Class) make) : callF(make);
  set(mc(), var, o);
  synchronized(fillVar_set) {} // sync?
  ret o;
}