// f : S -> Collection<S>
static Set<S> mapLike functionReturningList_transitiveHull_ciSet(O f, S input, O... _) {
  Set<S> seen = asCISet(optPar seen(_, linkedCISet()));
  exhaustIterator(repeatFunctionReturningList_iterator(f, input, paramsPlus(_, +seen)));
  ret seen;
}