sbool preload_done;

static void preload(S vars, O sow, O reap) {
  if (preload_done)
    callF(reap);
  else {
    final new Flag goOn;
    showButtons("Go on", r {
      hideControls(); // hide button again
      goOn.raise();
    });
    callF(sow);
    print("Preload done");
    goOn.waitUntilUp();
    Class c = hotwireDependent(programID());
    setOpt(c, preload_done := true);
    copyFields(mc(), c, splitAtSpace(vars));
    callMain(c);
  }
}