code: func (p *Pinger) Stop() {
  	p.debugln("Stop(): close(p.ctx.stop)")
  	close(p.ctx.stop)
  	p.debugln("Stop(): <-p.ctx.done")
  	<-p.ctx.done
  }
docs: // Stop stops RunLoop(). It must be called after RunLoop(). If not, it causes
  // panic.