sub comb_iter {
    my $items = shift || [];             # Think 1,2,3,4,5
    my $group = shift || 2;              # Think 2

    @$items>=$group or
       die "Insufficient elements ".scalar(@$items).
          " to make groups of $group\n";

    my @index=(0..$group-1);             # Think 0,1
    my @last=(@$items-$group..$#$items); # Think 3,4
    my @ret;

    while ($index[0]<=$last[0]) {        # Think first pass: 0 < 3
        push @ret,[@$items[@index]];

        # increment the last digit, and if we rollover carry it left
        unless (++$index[-1]<=$last[-1]) {

            # we've rolled over, we need to go left until
            # until the result after adding 1 is below the
            # respective last.
            my $pos=$#index;
            --$pos while ($pos and $index[$pos]+1>$last[$pos]);

            # increment the apropriate index, if we increment
            # $index[0] too high the loop exits because of the
            # while condition.
            my $v=++$index[$pos];

            # and then move right assigning the values in sequential order
            $index[++$pos]=++$v while $pos<$#index;
        }
    }
    return \@ret;
}