return fmt.Errorf("handler %v: %w", id.String(), ErrHandlerAlreadyExists)
		}

		m.handlers.single[id] = h
		return nil
	}
	subID := makeSubHandlerID(id, s)
	if m.handlers.hasSubhandler(subID) && !id.isTestHandler() {
		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.

}

func (h *handlers) hasAny(id HandlerID) bool {
	if !id.valid() {
		return false
	}
	return h.single[id] != nil || h.stateless[id] != nil || h.streams[id] != nil
}

func (h *handlers) hasSubhandler(id subHandlerID) bool {
	return h.subSingle[id] != nil || h.subStateless[id] != nil || h.subStreams[id] != nil
}

// RoundTripper provides an interface for type roundtrip serialization.
type RoundTripper interface {