static TreeSet mapToTreeSet(O f, Iterable l) {
  new TreeSet x;
  if (l != null) for (O o : l)
    x.add(callF(f, o));
  ret x;
}

static <A, B> TreeSet<B> lambdaMapLike mapToTreeSet(IF1<A, B> f, Iterable<A> l) {
  new TreeSet<B> x;
  if (l != null) for (o : l)
    x.add(f.get(o));
  ret x;
}

static <A, B> TreeSet<B> lambdaMapLike mapToTreeSet(Iterable<A> l, IF1<A, B> f) {
  ret mapToTreeSet(f, l);
}

Began life as a copy of #1011181