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