sub bfs_traverse {
    my ($nodefn, $tree, $depth) = @_;
    $depth ||= 0;
    $depth++;
    my @leaves   = grep !ref(), @$tree;
    my @branches = grep  ref(), @$tree;
    $nodefn->( $_, $depth ) for @leaves;
    bfs_traverse( $nodefn, [map @$_, @branches], $depth )
        if @branches;
}