code: static <S, E> GraphTraversal<S, E> union(Iterable<GraphTraversal<? super S, ? extends E>> traversals) {
          return union(__.identity(), traversals);
      }
docs: A type-safe way to do `__.union(a, b)`, as `Fragments.union(ImmutableSet.of(a, b))`.
  This avoids issues with unchecked varargs.