sub conflicts {
  my ($a, $b) = @_;
  my ($ai, $bi) = (0,0);
  my @conflicts;
  while (defined $a->[$ai] && defined $b->[$bi]) {
    my ($ac, $bc) = ($a->[$ai], $b->[$bi]);

    # high and low line numbers for chunks a and b
    my ($al, $ah) = ($ac->[0][1], $ac->[-1][1]);
    my ($bl, $bh) = ($bc->[0][1], $bc->[-1][1]);

    if ($ah < $bl) {       # chunk a precedes chunk b
      ++$ai;
    } elsif ($al > $bh) {  # chunk a follows chunk b
      ++$bi;
    } else {               # chunks overlap
      # compare the two chunks for conflicts
      my %h;
      for my $line (@$ac) {
        my ($s, $ln, $t) = @$line;
        $h{$ln}{$s} = $t;
      }
      for my $line (@$bc) {
        my ($s, $ln, $t) = @$line;
        next unless exists $h{$ln};
        if (exists $h{$ln}{$s} && $h{$ln}{$s} ne $t) {
          push @conflicts,
            [[$s, $ln, $h{$ln}{$s}], 
             [$s, $ln, $t]];
        }
      }

      ++$ai; ++$bi;
    }
  }
  return @conflicts;
}