static class JavaXClassLoader extends URLClassLoader {
  S progID;
  L<File> files;
  Set<Class> loadedClasses = synchroSet();
  
  *(S progID, L<File> files) {
    super(new URL[0]);
    this.progID = progID;
    this.files = files;
    
    ctex {
      for (File f : files)
        addURL(f.toURI().toURL());
    }
  }
  
  protected Class<?> findClass(S name) throws ClassNotFoundException {
    try {
      Class<?> c = super.findClass(name);
      loadedClasses.add(c);
      ret c;
    } catch (ClassNotFoundException e) {
      throw new ClassNotFoundException("Class " + name + " not found in " + joinWithComma(map(f f2s, files)) + " (progID=" + progID + ")");
    }
  }
}