static <A> A androidEvalInUIThread(final F0<A> f) {
  final new Var<A> v;
  final new Var<Throwable> error;
  final new Flag done;
  androidUI(r {
    try {
      v.set(callF(f));
    } catch e {
      error.set(e);
    } finally {
      done.raise();
    }
  });
  done.waitUntilUp();
  if (error.has()) throw rethrow(error!); else ret v!;
}