code: func TransformRefs(x interface{}, f func(Ref) (Value, error)) (interface{}, error) {
  	t := &GenericTransformer{func(x interface{}) (interface{}, error) {
  		if r, ok := x.(Ref); ok {
  			return f(r)
  		}
  		return x, nil
  	}}
  	return Transform(t, x)
  }
docs: // TransformRefs calls the function f on all references under x.