docs: public static boolean equalContents(int[][] xs, int[][] ys) {
      if(xs ==null)
        return ys == null;
      if(ys == null)
        return false;
      if(xs.length != ys.length)
        return false;
      for(int i = xs.length-1; i >= 0; i--) {
        if(! equalContents(xs[i],ys[i]))
          return false;
      }
      return true;
    }