sub process_files {
  my @stack=@_;

  my ($p,$dh,$path);
  while(@stack)
  {
    path = pop(@stack);
    opendir ($dh, $path) or die "Unable to open $path: $!\n";
    for( readdir($dh) )
    {
      next if( $_ eq '.' or $_ eq '..' );
      $p=$path.'/'.$_;
      if( -d $p )
      { push(@stack, $p); }
      elsif( substr($_,-4,4) eq '.pdw' )
      { push(@files_erg, $_); }
    }
    closedir ($dh);
  }
}