code: func (e *Engine) Close() {
  	// acquire mutex
  	e.mutex.Lock()
  	defer e.mutex.Unlock()
  
  	// stop acceptors
  	e.tomb.Kill(nil)
  	_ = e.tomb.Wait()
  }
docs: // Close will stop handling incoming connections and close all acceptors. The
  // call will block until all acceptors returned.
  //
  // Note: All passed servers to Accept must be closed before calling this method.