static long saveGZStructureToFile(S file, O o) {
  ret saveGZStructureToFile(getProgramFile(file), o);
}
  
// returns number of uncompressed bytes written
static long saveGZStructureToFile(File file, O o, structure_Data data default new) ctex {
  File parentFile = file.getParentFile();
  if (parentFile != null)
    parentFile.mkdirs();
  File tempFile = tempFileFor(file);
  if (tempFile.exists()) try {
    S saveName = tempFile.getPath() + ".saved." + now();
    copyFile(tempFile, new File(saveName));
  } catch (Throwable e) { printStackTrace(e); }
  
  FileOutputStream fileOutputStream = newFileOutputStream(tempFile.getPath());
  CountingOutputStream cos;
  try {
    GZIPOutputStream gos = new GZIPOutputStream(fileOutputStream);
    cos = new CountingOutputStream(gos);
    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(cos, "UTF-8");
    PrintWriter printWriter = new PrintWriter(outputStreamWriter);
    structureToPrintWriter(o, printWriter, data);
    printWriter.close();
    gos.close();
    fileOutputStream.close();
  } catch e {
    fileOutputStream.close();
    tempFile.delete();
    throw rethrow(e);
  }
  
  if (file.exists() && !file.delete())
    throw new IOException("Can't delete " + file.getPath());

  if (!tempFile.renameTo(file))
    throw new IOException("Can't rename " + tempFile + " to " + file);
    
  ret cos.getFilePointer();
}