static String commonPrefix(String a, String b) {

  int i = 0, nb = b.length(), na = a.length(), n = min(na, nb);

  while (i < n && a.charAt(i) == b.charAt(i))

    ++i;

  return i == nb ? b : substring(a, 0, i);

}

static S commonPrefix(L<S> l) {

  if (empty(l)) ret "";

  S s = first(l);

  for (int i = 1; i < l(l); i++) {

    if (empty(s)) ret s;

    s = commonPrefix(s, l.get(i)); // Yeah, cold be optimized

  }

  ret s;

}