static <A, B, C extends Iterable<B>> Set<B> concatMapToSet(Iterable<A> l, IF1<A, C> f) {
  new Set<B> set;
  if (l != null)
    for (a : l)
      addAll(set, f.get(a));
  ret set;
}

static <A, B, C extends Iterable<B>> Set<B> lambdaMapLike concatMapToSet(IF1<A, C> f, Iterable<A> l) {
  ret concatMapToSet(l, f);
}

static <A, B, C extends Iterable<B>> Set<B> lambdaMapLike concatMapToSet(IF1<A, C> f, A[] l) {
  new Set<B> set;
  if (l != null)
    for (a : l)
      addAll(set, f.get(a));
  ret set;
}

Began life as a copy of #1005492