cprint {
  /* // standard version
     static int ackermann(int a, int b) {
       if (a == 0) ret b+1;
       if (b == 0) ret ackermann(a-1, 1);
       ret ackermann(a-1, ackermann(a, b-1));
     }
  */

  // hibernatable version
  srecord ackermann(int a, int b) implements VStack.Computable<Int> {
    void step(VStack stack, O subComputationResult) {
      if (subComputationResult == null) {
        if (a == 0)
          stack._return(b+1);
        else if (b == 0)
          stack.tailCall(new ackermann(a-1, 1));
        else
          stack.push(new ackermann(a, b-1));
      } else
        stack.tailCall(new ackermann(a-1, subComputationResult/Int));
    }
  }
  
  start-thread {
    print("Result: " + vStackComputeWithStats(new ackermann(2, 2)));
    
    // detailed run
    VStack stack = new(new ackermann(2, 2));
    printStruct(stack);
    while ping (stack.step()) {
      printStruct(stack);
      stack = restructure(stack); // persist to string and re-create
    }
  }
}