static <A> HashSet<A> concatListsToSet(Collection<A>... lists) {
  new HashSet<A> l;
  for (Collection<A> list : lists)
    if (list != null)
      l.addAll(list);
  return l;
}

static <A> HashSet<A> concatListsToSet(Collection<? extends Collection<A>> lists) {
  new HashSet<A> l;
  for (Collection<A> list : lists)
    if (list != null)
      l.addAll(list);
  return l;
}

Began life as a copy of #1001241