code: func (cln *Client) WriteDescriptor(d *ble.Descriptor, b []byte) error {
  	rsp := cln.conn.sendReq(78, xpc.Dict{
  		"kCBMsgArgDeviceUUID":       cln.id,
  		"kCBMsgArgDescriptorHandle": d.Handle,
  		"kCBMsgArgData":             b,
  	})
  	return rsp.err()
  }
docs: // WriteDescriptor writes a characteristic descriptor to a server. [Vol 3, Part G, 4.12.3]