sbool preload_done;

static void preload(S vars, O sow, O reap) {

  if (preload_done)

    callF(reap);

  else {

    final new Flag goOn;

    final Var<S> nextProgramID = new(programID());

    showButtons("Go on", r {

      hideControls(); // hide button again

      goOn.raise();

    }, "Go on as...", r {

      selectSnippetID(voidfunc(S snippetID) {

        hideControls();

        nextProgramID.set(snippetID);

        goOn.raise();

      });

    });

    callF(sow);

    print("Preload done");

    goOn.waitUntilUp();

    Class c = hotwireDependent(nextProgramID!);

    setOpt(c, preload_done := true);

    copyFields(mc(), c, splitAtSpace(vars));

    callMain(c);

  }

}