reactfx/src/main/java/org/reactfx/util/Lists.java
public static <E> List<E> concatView(List<List<? extends E>> lists) {
        if(lists.isEmpty()) {
            return Collections.emptyList();
        } else {
            return ConcatView.create(lists);
        }
    }
public static <E> List<E> concatView(List<List<? extends E>> lists) {
        if(lists.isEmpty()) {
            return Collections.emptyList();
        } else {
            return ConcatView.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.
The returned list is unmodifiable. Modifications to underlying lists
will be visible through the concatenation view.