static int compareIgnoreCase_lower(S s1, S s2) {
  if (s1 == null) ret s2 == null ? 0 : -1;
  if (s2 == null) ret 1;
  
  int n1 = s1.length();
  int n2 = s2.length();
  int min = Math.min(n1, n2);
  for (int i = 0; i < min; i++) {
    char c1 = s1.charAt(i);
    char c2 = s2.charAt(i);
    if (c1 != c2) {
      c1 = Character.toLowerCase(c1);
      c2 = Character.toLowerCase(c2);
      if (c1 != c2) ret c1-c2;
    }
  }
  ret n1-n2;
}