code: func (b *Bus) Subscribe(h Handler) bool {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  
  	for _, v := range b.handlers {
  		if v == h {
  			return false
  		}
  	}
  
  	b.handlers = append(b.handlers, h)
  	return true
  }
docs: // Subscribe to events.
  // It will return true if successfully subscribed, means the handler haven't subscribed before.