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{}{}
  	}
  	err := h.validateHelper(key, helper)
  	if err != nil {
  		return errors.WithStack(err)
  	}
  	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:
  /*
  	func(...) (string) {}
  	func(...) (string, error) {}
  	func(...) (template.HTML) {}
  	func(...) (template.HTML, error) {}
  */