code: func (mux *Mux) Listen(hdrs []byte) net.Listener {
  	// Create new handler.
  	h := mux.handler()
  
  	// Register each header byte.
  	for _, hdr := range hdrs {
  		// Create a new listener and assign it.
  		mux.handlers[hdr] = h
  	}
  
  	return h
  }
docs: // Listen returns a listener that receives connections from any byte in hdrs.
  // Re-registering hdr bytes will overwrite existing handlers.