code: func (tpl *Template) RegisterHelper(name string, helper interface{}) {
  	tpl.mutex.Lock()
  	defer tpl.mutex.Unlock()
  
  	if tpl.helpers[name] != zero {
  		panic(fmt.Sprintf("Helper %s already registered", name))
  	}
  
  	val := reflect.ValueOf(helper)
  	ensureValidHelper(name, val)
  
  	tpl.helpers[name] = val
  }
docs: // RegisterHelper registers a helper for that template.