static S longestCommonSubstring(S s1, S s2) {

  int Start = 0;

  int Max = 0;

  int l1 = l(s1), l2 = l(s2);

  for ping (int i = 0; i < l1; i++) {

    for ping (int j = 0; j < l2; j++) {

      int x = 0;

      while (s1.charAt(i + x) == s2.charAt(j + x)) {

        x++;

        if ((i + x) >= l1 || (j + x) >= l2) break;

      }

      if (x > Max) {

        Max = x;

        Start = i;

      }

    }

  }

  ret s1.substring(Start, Start + Max);

}