docs: public static int nthOccurrenceOf(final String s, final char needle, int n) {
      checkNotNull(s);
      checkArgument(n > 0);
      for (int i = 0; i < s.length(); ++i) {
        if (needle == s.charAt(i)) {
          --n;
          if (n == 0) {
            return i;
          }
        }
      }
      return -1;
    }