code: public static <E> List<E> concat(List<List<? extends E>> lists) {
          return ListConcatenation.create(lists);
      }
docs: Returns a list that is a concatenation of the given lists. The returned
  list is a view of the underlying lists, without copying the elements.
  As opposed to {@link #concatView(List)}, the underlying lists must not
  be modified while the returned concatenation view is in use. On the other
  hand, this method guarantees balanced nesting if some of the underlying
  lists are already concatenations created by this method.