static File renameFileBaseName(File f, S newBaseName) {
  S ext = fileExtension(a);
  S base = dropSuffix(ext, fileName(a));
  File f2 = fileInSameDir(f, newBaseName + ext);
  if (eq(f, f2)) ret f;
  if (renameFile(f, f2)) null;
  ret f2;
}