docs: @SuppressWarnings("unchecked")
    protected <T> T set(DBIDRef id, int index, T value) {
      Object[] d = data.get(DBIDUtil.deref(id));
      if(d == null) {
        d = new Object[rlen];
        data.put(DBIDUtil.deref(id), d);
      }
      T ret = (T) d[index];
      d[index] = value;
      return ret;
    }