sub r {
    my $g = shift;
    my $v = shift;
    my @ns = $g->neighbors($v);
    print $v."\n";
    $g->delete_vertex($v);
    foreach my $n (@ns){
        r($g, $n);
    }
}

while (scalar $g->vertices > 0){
    print "next set:\n";
    r($g, ($g->vertices)[0]);
}