reactfx/src/main/java/org/reactfx/util/Lists.java
public static <E> List<E> concat(List<List<? extends E>> lists) {
        return ListConcatenation.create(lists);
    }
public static <E> List<E> concat(List<List<? extends E>> lists) {
        return ListConcatenation.create(lists);
    }
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.