sub permutation {
	my ($prob, $start) = (1, shift);
	$prob *= $start-- foreach (1 .. $_[0]);
	$prob;
}

##</code><code>##

sub permutation {
	my $prob = 1;
	$prob *= $_ foreach ((($_[0]+1) - $_[1]) .. $_[0]);
	return $prob;
}

##</code><code>##

sub permutation {
	my $prob = 1;
	return (map { $prob *= $_ } (($_[0] - $_[1] + 1) .. $_[0]))[-1];
}