code: func WalkRefs(x interface{}, f func(Ref) bool) {
  	vis := &GenericVisitor{func(x interface{}) bool {
  		if r, ok := x.(Ref); ok {
  			return f(r)
  		}
  		return false
  	}}
  	Walk(vis, x)
  }
docs: // WalkRefs calls the function f on all references under x. If the function f
  // returns true, AST nodes under the last node will not be visited.