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

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