code: func (cln *Client) ReadDescriptor(d *ble.Descriptor) ([]byte, error) {
  	rsp := cln.conn.sendReq(77, xpc.Dict{
  		"kCBMsgArgDeviceUUID":       cln.id,
  		"kCBMsgArgDescriptorHandle": d.Handle,
  	})
  	if rsp.err() != nil {
  		return nil, rsp.err()
  	}
  	return rsp.data(), nil
  }
docs: // ReadDescriptor reads a characteristic descriptor from a server. [Vol 3, Part G, 4.12.1]