sub diff
{
	my $a = shift;	# array ref
	my $b = shift;	# array ref
	my $retval = [];
	my $hunk = [];
	my $discard = sub { push( @$hunk, [ '-', $_[ 0 ], $a->[$_[0]] ] ) };
	my $add     = sub { push( @$hunk, [ '+', $_[ 1 ], $b->[$_[1]] ] ) };
	my $match   = sub { push( @$retval, $hunk ) if scalar(@$hunk); $hunk = [] };
	traverse_sequences( $a, $b,
		{ MATCH     => $match, 
                  DISCARD_A => $discard, 
                  DISCARD_B => $add 
                }, @_ );
	&$match();
	return wantarray ? @$retval : $retval;
}