code: func ensureValidHelper(name string, funcValue reflect.Value) {
  	if funcValue.Kind() != reflect.Func {
  		panic(fmt.Errorf("Helper must be a function: %s", name))
  	}
  
  	funcType := funcValue.Type()
  
  	if funcType.NumOut() != 1 {
  		panic(fmt.Errorf("Helper function must return a string or a SafeString: %s", name))
  	}
  
  	// @todo Check if first returned value is a string, SafeString or interface{} ?
  }
docs: // ensureValidHelper panics if given helper is not valid