static Steppable combineSteppables(Iterable<? extends Steppable> steppables) {
  ret iteratorToSteppable(roundRobinCombinedIterator(map steppableToIterator(steppables)));
}

static <A extends Steppable> Steppable combineSteppables(A... steppables) {
  ret combineSteppables(asList(steppables));
}