static LS startingWith_drop(Collection<S> l, fS prefix) {
  new L<S> out;
  for (S s : unnull(l))
    if (startsWith(s, prefix))
      out.add(substring(s, l(prefix)));
  ret out;
}

static LS startingWith_drop(S prefix, Collection<S> l) {
  ret startingWith_drop(l, prefix);
}

Began life as a copy of #1018727