static Set<Char> charactersAppearingExactlyOnceInEachString(L<S> l) {
  Set<Char> set = null;
  for (S s : l) {
    Set<Char> newSet = keysWithValue(1, asMap(asMultiSet(charsAsList(s))));
    if (set != null) newSet = intersectSets(set, newSet);
    set = newSet;
  }
  ret set;
}