code: public void scheduleOnce(Envelope envelope, long time) {
          if (envelope.getMailbox() != this) {
              throw new RuntimeException("envelope.mailbox != this mailbox");
          }
  
          envelopes.putEnvelopeOnce(envelope, time, comparator);
      }
docs: Send envelope once at time
  
  @param envelope envelope
  @param time     time