cprint {
  sclass f_ackermann implements VStack.Computable<BigInt> {
    BigInt a, b;
    int currentStep; // 0 in the beginning, goes up to 1

    *(BigInt *a, BigInt *b) {}
    
    void step(VStack stack, O subComputationResult) {
      if (currentStep == 0) {
        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)));
          currentStep = 1;
        }
      } else if (currentStep == 1)
        stack.tailCall(new f_ackermann(minus(a, BigInt.ONE), subComputationResult/BigInt));
      else fail();
    }
  }
  
  start-thread {
    print("Result: " + vStackComputeWithStats(new f_ackermann(bigInt(2), bigInt(2))));
  }
}

Began life as a copy of #1030375