sub AUTOLOAD {
  our $AUTOLOAD;
  my $s = shift;
  $AUTOLOAD  =~ /.*::get(_next)*(_\w+)*_files*$/ or
    croak "No such method: $AUTOLOAD";
  my ($next, $type) = ($1, $2);

  if ($next) {                                      # create an iterator
    my $class = ref($s) . '::Iterator';
    if (!$type) {                                   
      return $class->new($s->get_files);            # iterator for all files
    } else {
      return $class->new(@{$s->{$type.'_files'}});  # iterator for file subset
    }
  } else {                                        
    return @{$s->{$type.'_files'}};                 # return file subset
  }
}