sub mixedgray_iterator {
    my @domains = @_;
    my @sizes = map { scalar @$_ } @domains;
    die "all items must have at least two positions"
        if grep {$_<2} @sizes;
    my @cur_indices = (0) x @domains;
    my @pos_to_loop_next = 0 .. @domains;
    my @dir = (1) x @domains;
    my $done;
    return sub {
        return undef if $done;
        my $rv = [map {$domains[$_][$cur_indices[$_]]} 0..$#domains];
        if ($pos_to_loop_next[0]==@domains) { $done=1; return $rv }
        (my $domain,$pos_to_loop_next[0]) = ($pos_to_loop_next[0], 0);
        $cur_indices[$domain] += $dir[$domain];
        if ($cur_indices[$domain]==0 || $cur_indices[$domain]==$sizes[$domain]-1) {
            $dir[$domain] = -$dir[$domain];
            $pos_to_loop_next[$domain] = $pos_to_loop_next[$domain+1];
            $pos_to_loop_next[$domain+1] = $domain+1;
        }
        return $rv;
    }
}