code: func WalkClosures(x interface{}, f func(interface{}) bool) {
  	vis := &GenericVisitor{func(x interface{}) bool {
  		switch x.(type) {
  		case *ArrayComprehension, *ObjectComprehension, *SetComprehension:
  			return f(x)
  		}
  		return false
  	}}
  	Walk(vis, x)
  }
docs: // WalkClosures calls the function f on all closures under x. If the function f
  // returns true, AST nodes under the last node will not be visited.