// async fibonacci
function async_fib(x) { return spawn(function*() {
  throw new Task.Result(
    x < 2 ? x 
          : yield async_fib(x-1) + yield async_fib(x-2));
});}

// with fibers / awaits:
async int async_fib(int x) {
  return x < 2 ? x 
               : await async_fib(x-1) + await async_fib(x-2);
}