use warnings;
use strict;

die "Usage: $0 INFILE\n" unless @ARGV==1;
my $INFILE = shift @ARGV;

open my $ofh, '|-', 'tac' or die "tac (out): $!";
open my $ifh, '-|', 'tac', $INFILE or die "tac $INFILE: $!";
my ($aminblock,$prevnum,$foundstr);
my %found;
while (<$ifh>) {
	chomp;
	my $out=1;
	if (!$aminblock) {
		if (/^end$/) { undef $foundstr; $aminblock=1 }
		elsif (/^\s*(?:foo\s+)?ref\s+(\d+)\s*$/) {
			die "ref $1 without block?" unless exists $found{$1};
			$out = !$found{$1};
		}
		else { die "unexpected outside of a block: $_" }
	}
	else {
		if (/^\s*(\d+)\s*$/) { $prevnum=$1 }
		elsif (/^begin$/) {
			die "block ended without number?" unless defined $prevnum;
			$found{$prevnum} = $foundstr;
			undef $prevnum;
			$aminblock=0;
		}
		else {
			undef $prevnum;
			if (/bar/) { $foundstr=1 }
		}
	}
	print {$ofh} $_, "\n" if $out;
}
close $ifh or die "tac $INFILE: ".($!||"\$?=$?");
close $ofh or die "tac (out): ".($!||"\$?=$?");