sub compare {
  my @refs = @_;
  my ($all, $inter, $diff);
  my %files;
  foreach my $ref (@refs) {
    foreach (@$ref) {
      $files{$_}++;
    }
  }
  $all = [ keys %files ];
  foreach my $e (@$all) {
    push @{ $files{$e} > 1 ? $inter : $diff }, $e;
  }
  return ($all, $inter, $diff);
}