code: protected final void writekv(String key, Object value) throws IOException {
      String valStr = value == null ? "null" : value.toString();
      if (valStr.contains("\n"))
        throw new IOException("The `value` must not contain newline characters, got: " + valStr);
      if (lkv.containsKey(key))
        throw new IOException("Key " + key + " was already written");
      lkv.put(key, valStr);
    }
docs: Write a simple value to the model.ini/[info] section. Here "simple" means a value that can be stringified with
  .toString(), and its stringified version does not span multiple lines.