my %seen = ();
my %xeen = ();

sub op {
	my $a = shift;
	my $b = shift;
	return $a ^ $b;
}

my $min = 1;
sub a {
	my $prev = shift;
	$seen{$prev}++;
	while ($seen{$min}) {
		$min++;
	}
	my $a = $min;
	while ($seen{$a} || $xeen{op($a, $prev)}) {
		$a++;
	}
	$xeen{op($a, $prev)}++;
	return $a;
}

my $a = 1;
foreach my $n (1..10_000) {
	print "$n $a\n";
	$a = a($a);
}