code: func (v *evalVisitor) callHelper(name string, helper reflect.Value, node *ast.Expression) interface{} {
  	result := v.callFunc(name, helper, v.helperOptions(node))
  	if !result.IsValid() {
  		return nil
  	}
  
  	// @todo We maybe want to ensure here that helper returned a string or a SafeString
  	return result.Interface()
  }
docs: // callHelper invoqs helper function for given expression node