static File renameFileToSomeBackupName(File f) {
  ret renameFileToSomeBackupName(f, "backup");
}

static File renameFileToSomeBackupName(File f, S code) {
  File f2 = fileInSameDir(f, f.getName() + "." + code + "." + ymd_minus_hms());
  renameFile_assertTrue(f, f2);
  ret f2;
}