static S commonPrefixCI(S a, S b) {

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

  while (i < n && eqic(a.charAt(i), b.charAt(i)))

    ++i;

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

}

static S commonPrefixCI(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 = commonPrefixCI(s, l.get(i));

  }

  ret s;

}