return fmt.Errorf("handler %v, subroute:%v: %w", id.String(), s, ErrHandlerAlreadyExists)
	}
	m.handlers.subSingle[subID] = h
	// Copy so clients can also pick it up for other subpaths.
	m.handlers.subSingle[makeZeroSubHandlerID(id)] = h
	return nil
}

/*
// RegisterStateless will register a stateless handler that serves
// []byte -> stream of ([]byte, error) requests.

	s[0] = byte(id)
	s[1] = 0 // Reserved
	return s
}

func (s *subHandlerID) String() string {
	if s == nil {
		return ""
	}
	return hex.EncodeToString(s[:])
}

func makeZeroSubHandlerID(id HandlerID) subHandlerID {
	return subHandlerID{byte(id)}
}

type handlers struct {
	single    [handlerLast]SingleHandlerFn
	stateless [handlerLast]*StatelessHandler

	if !h.valid() {
		return nil, ErrUnknownHandler
	}
	if c.State() != StateConnected {
		return nil, ErrDisconnected
	}
	handler := c.handlers.subStreams[makeZeroSubHandlerID(h)]
	if handler == nil {
		if debugPrint {
			fmt.Println("want", makeZeroSubHandlerID(h), c.route, "got", c.handlers.subStreams)
		}
		return nil, ErrUnknownHandler
	}

	var requests chan []byte
	var responses chan Response