sub combo (Int $by is copy, @list is copy) returns Ref {
    my @position = 0 .. ($by - 2), $by - 2;
    my @stop     = @list.elems - $by .. @list.end;
    my $done     = undef;

    gather {
        until $done {
            my $cur = @position.end;
            while ++@position[$cur] > @stop[$cur] {
                --$cur;
                @position[$cur]++;
                next if @position[$cur] > @stop[$cur];
                my $new_pos = @position[$cur];
                @position[$cur .. @position.end] = $new_pos .. ($new_pos + $by);
                last;
            }
            $done = 1 if @position[0] == @stop[0]; 
            take [ @list[@position] ];
        }
    }
}