code: func (eventStream *EventStream) Subscribe(name string, handler eventHandler) error {
  	eventStream.handlersLock.Lock()
  	defer eventStream.handlersLock.Unlock()
  
  	if _, ok := eventStream.handlers[name]; ok {
  		return fmt.Errorf("handler %s already exists", name)
  	}
  
  	eventStream.handlers[name] = handler
  	return nil
  }
docs: // Subscribe adds the handler to be called into EventStream