sclass _Activity {
  long started;
  O r;
  Thread thread;
}

static Set<_Activity> tempActivity_list = synchroHashSet();
sbool tempActivity_debug;

static AutoCloseable tempActivity(fO r) {
  if (tempActivity_debug) print("Activity started: " + r);
  final new _Activity a;
  a.started = sysNow();
  a.r = r;
  a.thread = currentThread();
  tempActivity_list.add(a);
  ret new AutoCloseable {
    public void close() {
      tempActivity_list.remove(a);
      if (tempActivity_debug) {
        int n = l(tempActivity_list);
        print("Activity ended: " + r
          + (n == 0 ? "" : " - " + n + " remaining"));
      }
    }
  };
}

Began life as a copy of #1014792