/**
  * @return boolean
  */
 public function addPermission(User_ForgeUGroup $user_group, User_ForgeUserGroupPermission $permission)
 {
     $user_group_id = $user_group->getId();
     $permission_id = $permission->getId();
     if (!$this->permissions_dao->permissionExistsForUGroup($user_group_id, $permission_id)) {
         return $this->permissions_dao->addPermission($user_group_id, $permission_id);
     }
     return true;
 }