static <A> Set<A> filterSet(Iterable<A> c, IPred<A> pred) {
  Set<A> x = similarEmptySetOrLinkedHashSet(c);
  if (c != null) for (A o : c)
    if (pred.get(o))
      x.add(o);
  ret x;
}

static <A> Set<A> lambdaMapLike filterSet(IPred<A> pred, Iterable<A> c) {
  ret filterSet(c, pred);
}

Began life as a copy of #1021399