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