sub wait_for_files {
   my $files = shift;

   my %not_changed_yet = %$files;
   while () {
      last if (! %not_changed_yet);
      for my $file (keys(%$files)) {
         if (-s $files->{$file}) { # file has .prd
            delete $not_changed_yet{$file};
         }
      }
   }
}