code: func NewEchoChar() *ble.Characteristic {
  	e := &echoChar{m: make(map[string]chan []byte)}
  	c := ble.NewCharacteristic(EchoCharUUID)
  	c.HandleWrite(ble.WriteHandlerFunc(e.written))
  	c.HandleNotify(ble.NotifyHandlerFunc(e.echo))
  	c.HandleIndicate(ble.NotifyHandlerFunc(e.echo))
  	return c
  }
docs: // NewEchoChar ...