static Set<Char> commonCharactersInStrings(L<S> l) {
  Set<Char> set = null;
  for (S s : l) {
    new Set<Char> newSet;
    for i over s: {
      char c = s.charAt(i);
      if (set == null || set.contains(c))
        newSet.add(c);
    }
    set = newSet;
  }
  ret set;
}