Yet another Ackermann function impl.

ack.pl6

use v6;
sub ack(Int $m is copy, Int $n is copy) {
	my @stack;
	loop {
		if $m > 3 {
			--$m;
			push @stack, ($m xx $n).Slip;
			--$m;
			push @stack, (3 .. $m).reverse.Slip;
			$n = 13;
		} elsif $m == 3 {
			$n = 2 ** ( $n + 3 ) - 3;
		} elsif $m == 2 {
			$n = 2 * $n + 3;
		} elsif $m >= 0 {
			$n = $n + $m + 1
		} else {
			fail "negative m!";
		}
		return $n unless @stack;
		$m = pop @stack;
	}
}

say ack(4,1);
say .chars, " digits starting with ", .substr(0,50), "..." given ack(4,2);
say "Took {now - INIT {now}} seconds."