int main(int argc, char **argv) {
  // # general FLTK initialization w/o Fl::lock()
  Fl_Double_Window* w = new Fl_Double_Window(200, 200, "Single Thread");
  // # window initialization goes here
  w->end();
  w->show();
  browser1->add("Prime numbers:");
  browser2->add("Prime numbers:");

  // Enable multi-thread support by locking from the main thread.
  Fl::lock();

  // # Start (child) threads AFTER Fl::lock() ...

  // One thread displaying in one browser
  fl_create_thread(prime_thread, prime_func, browser1);
  // Several threads displaying in another browser
  fl_create_thread(prime_thread, prime_func, browser2);
  // # start more threads.

  // # You could do even more FLTK initializations here
  // # w/o interfering with the child threads because
  // # the FLTK lock is held...

  // # WE MUST NOT release the lock because otherwise
  // # the child threads could acquire the lock before
  // # the FLTK event loop gets started (race condition).

  Fl::run();
  return 0;
}