sub new_tree
  {
    my $class = shift;
    my $id = shift;
    my $depth = shift;
    my $self = $class->new($id);
    $self->{_depth} = $depth;
    $self->{_sub_cats} = [
       sort { $a->name cmp $b->name }
       map { $class->new_tree($_, $depth+1) }
       $self->sub_cat_ids
    ];
    return $self;
  }

  sub sub_cats { @{shift->{_sub_cats} }