// TODO: either put ExternalMutex in JavaX
// or make externalMutexMap local
static IAutoCloseableF0 tempExternalMutexFor(O o) {
  var map = externalMutexMap();
  synchronized(map) {
    ExternalMutex mutex = map.get(o);
    if (mutex == null)
      map.put(o, mutex = new ExternalMutex);
    var _mutex = mutex;
    mutex.incRefCount();
    ret new IAutoCloseableF0 {
      public O get() { ret _mutex; }
      public void close {
        synchronized(map) {
          if (_mutex.decRefCount() == 0)
            map.remove(o);
        }
      }
    };
  }
}

Began life as a copy of #1031844