save_xkcd( 'outdir', 'startingid' );
sub save_xkcd {
    $starting_id ||= id_from_progress();
    my @ids = $starting_id;
    while( @ids ) {
       my $cid = shift @ids;
       my $page = sptintf '...%s', $cid;
       $mech->get( $page );
       save_stuff( $mech, $cid );
       next_page( $mech , \@ids );
       maybe_sleep();
    }
}