code: func (cln *Client) ReadCharacteristic(c *ble.Characteristic) ([]byte, error) {
  	rsp := cln.conn.sendReq(65, xpc.Dict{
  		"kCBMsgArgDeviceUUID":                cln.id,
  		"kCBMsgArgCharacteristicHandle":      c.Handle,
  		"kCBMsgArgCharacteristicValueHandle": c.ValueHandle,
  	})
  	if rsp.err() != nil {
  		return nil, rsp.err()
  	}
  	return rsp.data(), nil
  }
docs: // ReadCharacteristic reads a characteristic value from a server. [Vol 3, Part G, 4.8.1]