sub choose_n_iter {
    my @todo = [ shift, [ @_ ], [] ];
    return sub {
        while ( @todo ) {
            my ( $n, $pool, $tally ) = @{ shift @todo };

            return $tally if $n == 0;

            next if @$pool == 0;
            
            my ( $first, @rest ) = @$pool;

            push @todo, [ $n - 1, \@rest, [ @$tally, $first ]],
                        [ $n    , \@rest,    $tally          ];
                        
        }
        return;
    };
}