# Given a list of M items and a number N,
# generate all size-N subsets of M
sub choose_n {
    my $n = pop;
    # Base cases
    return []   if $n == 0  or $n > @_;
    return [@_] if $n == @_;
    # otherwise..
    my ($first, @rest) = @_;
    # combine $first with all N-1 combinations of @rest,
    # and generate all N-sized combinations of @rest
    my @include_combos = choose_n(@rest, $n-1);
    my @exclude_combos = choose_n(@rest, $n);
    return ( (map {[$first, @$_]} @include_combos)
	     , @exclude_combos );
}

##</code><code>##

sub iter_choose_n {
    my $n = pop;
    # Base cases
    my $once = 0;
    return sub {$once++ ? () : []} if $n == 0  or $n > @_;
    my ($first, @rest) = @_;
    return sub {$once++ ? () : [$first, @rest]} if $n == @_;

##</code><code>##

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
    my $exclude_iter = iter_choose_n(@rest, $n);
    return sub {
        if (my $set = $include_iter->()) {
            return [$first, @$set];
        }
        else {
            return $exclude_iter->();
        }
    }

##</code><code>##

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
    my $exclude_iter;
    return sub {
        if (my $set = $include_iter->()) {
            return [$first, @$set];
        }
        else {
            $exclude_iter ||= iter_choose_n(@rest, $n);
            return $exclude_iter->();
        }
    }
}

##</code><code>##

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
    my $exclude_iter;
    return sub {
        if ($include_iter and my $set = $include_iter->()) {
            return [$first, @$set];
        }
        else {
            if ($include_iter) {
                undef $include_iter;
                $exclude_iter = iter_choose_n(@rest, $n);
            }
            return $exclude_iter->();
        }
    }
}