docs: public static String format(double[][] m, String pre) {
      StringBuilder output = new StringBuilder() //
          .append(pre).append("[\n").append(pre);
      for(int i = 0; i < m.length; i++) {
        formatTo(output.append(" ["), m[i], ", ").append("]\n").append(pre);
      }
      return output.append("]\n").toString();
    }