static File findMainJavaInDir(File dir) {
  File f = newFile(dir, "main.java");
  if (f.exists()) ret f;
  for (File ff : listFiles(dir)) {
    S s = ff.getName();
    if (isInteger(dropPrefixSuffix_orNull("m", ".java", ff.getName())))
      ret ff;
  }
  null;
}