push @{$groups[@$_]}, $_ for @tails;
    @groups = grep {defined $_} @groups;
    
    for my $group (@groups) {
        my @ref = @{$group->[-1]};
        my @org = @ref;
        my $count = 1;
        
        pop @$group;
        
        while (@$group) {
            my @new = @{pop @$group};
            my @diffs = Algorithm::Diff::diff (\@ref, \@new);
            
            for my $change (@diffs) {
                next unless $change->[0][0] eq '-';
                $ref[$change->[0][1]] = undef;
            }

            ++$count;
        }

        for (0 .. $#ref) {
            next if defined $ref[$_];
            $org[$_] = '*****';
        }
        
        push @{$context->{digest}}, [join (' ', @org), $count];
    }