public function checkOverlap($task = null)
{
if ($task == null && $this->task == null) {
throw new Task_Not_Found();
}
if ($task == null) {
$task = $this->task;
}
$goal = new Goal();
$goal->where(array("start <=" => $this->start, "end >=" => $this->end))->get_by_related($task);
if ($goal->exists()) {
throw new Goal_Overlap();
}
}
