cprint {
  srecord f_ackermann(BigInt a, BigInt b) implements VStack.Computable<BigInt> {
    void step(VStack stack, O subComputationResult) {
      if (!stack.hasSubResult()) {
        if (a.equals(BigInt.ZERO))
          stack._return(plus(b, BigInt.ONE));
        else if (b.equals(BigInt.ZERO))
          stack.tailCall(new f_ackermann(a.subtract(BigInt.ONE), BigInt.ONE);
        else
          stack.push(new f_ackermann(a, minus(b, BigInt.ONE)));
      } else
        stack.tailCall(new f_ackermann(minus(a, BigInt.ONE), subComputationResult/BigInt));
    }
  }
  
  start-thread {
    print("Result: " + vStackComputeWithStats(new f_ackermann(bigInt(2), bigInt(2))));
    
    // detailed run
    VStack stack = new(new f_ackermann(bigInt(2), bigInt(2)));
    printStruct(stack);
    while ping (stack.step()) {
      printStruct(stack);
      stack = restructure(stack); // persist to string and re-create
    }
  }
}