The other approach is to use the isSuperset() method, which determines if the argument is a subset of the collection where we're calling the method:

const myList = List.of(
  List.of(1, 2, 3),
  List.of(4, 5, 6),
  List.of(7, 8, 9)
);
const isSuperset = myList
  .flatten()
  .isSuperset(List.of(2, 5, 8));

console.log('isSuperset', isSuperset);
// -> isSuperset true

As you can see, this is just another way of getting the same result as the prior example.