code: func (h *HelperMap) Add(key string, helper interface{}) error {
  	h.moot.Lock()
  	defer h.moot.Unlock()
  	if h.helpers == nil {
  		h.helpers = map[string]interface{}{}
  	}
  	h.helpers[key] = helper
  	return nil
  }
docs: // Add a new helper to the map. New Helpers will be validated to ensure they
  // meet the requirements for a helper: