docs: private void doDelete(DBIDRef id) {
      // Remove id
      ids.remove(id);
      // Remove from all representations.
      for(Relation<?> relation : relations) {
        // ID has already been removed, and this would loop...
        if(relation == idrep) {
          continue;
        }
        if(!(relation instanceof ModifiableRelation)) {
          throw new AbortException("Non-modifiable relations have been added to the database.");
        }
        ((ModifiableRelation<?>) relation).delete(id);
      }
      DBIDFactory.FACTORY.deallocateSingleDBID(id);
    }