static File findCmdOnPATH(S cmd) {
  S path = System.getenv("PATH");
  L<S> dirs = splitAt(path, File.pathSeparator);
  S c = isWindows() ? addSuffix(cmd, ".exe") : cmd;
  for (S dir : dirs) {
    File f = new File(dir, c);
    if (f.isFile()) ret f;
  }
  null;
}