sub series(@items, $recursion_level = 2) {
    return if $recursion_level < 0;
    return if @items < 2;
    
    # are all items the same?
    return @items[0] if [==] @items;

    # detect arithmetic sequences
    my @diffs = map { @items[$_+1] - @items[$_] }, 0 .. (@items - 2);
    my $d = series(@diffs, $recursion_level - 1);
    return @items[*-1] + $d if $d.defined;

    # detect geometric sequences
    my $r = try {
        my @ratios = map { @items[$_+1] / @items[$_] }, 0 .. (@items - 2);
        series(@ratios, $recursion_level - 1);
    }
    return @items[*-1] * $r if $r.defined;

    # give up
    return;
}