if err != nil {
		return err
	}
	if msg.Flags&FlagSubroute != 0 {
		if m.subroute == nil {
			return fmt.Errorf("internal error: subroute not defined on client")
		}
		hid := m.subroute.withHandler(msg.Handler)
		before := len(dst)
		dst = append(dst, hid[:]...)
		if debugPrint {
			fmt.Println("Added subroute", hid.String(), "to message", msg, "len", len(dst)-before)
		}
	}

func makeSubHandlerID(id HandlerID, subRoute string) subHandlerID {
	b := subHandlerID(sha256.Sum256([]byte(subRoute)))
	b[0] = byte(id)
	b[1] = 0 // Reserved
	return b
}

func (s subHandlerID) withHandler(id HandlerID) subHandlerID {
	s[0] = byte(id)
	s[1] = 0 // Reserved
	return s
}

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

pkg net/http/httputil, type ReverseProxy struct, Director //deprecated #73161
pkg net/netip, method (Prefix) Compare(Prefix) int #61642
pkg os, method (*Process) WithHandle(func(uintptr)) error #70352
pkg os, var ErrNoHandle error #70352
pkg reflect, method (Value) Fields() iter.Seq2[StructField, Value] #66631
pkg reflect, method (Value) Methods() iter.Seq2[Method, Value] #66631