code: func (c *conn) subscribed(char *ble.Characteristic) {
  	h := char.Handle
  	if _, found := c.notifiers[h]; found {
  		return
  	}
  	send := func(b []byte) (int, error) {
  		c.dev.sendCmd(c.dev.pm, 15, xpc.Dict{
  			"kCBMsgArgUUIDs":       [][]byte{},
  			"kCBMsgArgAttributeID": h,
  			"kCBMsgArgData":        b,
  		})
  		return len(b), nil
  	}
  	n := ble.NewNotifier(send)
  	c.notifiers[h] = n
  	req := ble.NewRequest(c, nil, 0) // convey *conn to user handler.
  	go char.NotifyHandler.ServeNotify(req, n)
  }
docs: // server (peripheral)