@Override
  public int compareTo(Tache t) {
    int c = _date.compareTo(t._date);
    if (c != 0) return c;
    boolean b1 = _begin.isNull(), b2 = t._begin.isNull();
    if (b1)
      c = b2 ? 0 : 1;
    else
      c = b2 ? -1 : _begin.compareTo(t._begin);
    if (c != 0) return c;
    b1 = _end.isNull();
    b2 = t._end.isNull();
    if (b1)
      c = b2 ? 0 : -1;
    else
      c = b2 ? 1 : _end.compareTo(t._end);
    if (c != 0) return c;
    /*
    if (_begin.isNull())   return t._begin.isNull() ? 0 : 1;
    if (t._begin.isNull()) return -1;
    c = _begin.compareTo(t._begin);
    if (c != 0) return c;
    c = _end.compareTo(t._end);
    if (c != 0) return c;
    */
    return Priorite.compare(_priority, t._priority);
  }