return receivers.contains(r);
lock.writeLock().acquire(); acquired = true; } catch (InterruptedException e) { throw new RuntimeException("Failed to obtain lock", e); } try { return receivers.contains(r); } finally { if (acquired) { lock.writeLock().release(); }
   public boolean contains(Receiver r)   {      synchronized(receivers)      {         return receivers.contains(r);      }   }