// Goal: Java code with backtracking, like this, to return 4 different values:

// Color color = Color.black or Color.white;

// S shape = "circle" or "square";

// return "A " + color + " " + shape;

// 1. Simple version without backtracking

embedded S functionSimple() {

  S color = "black";

  S shape = "circle";

  return "A " + color + " " + shape;

}

//ret functionSimple();

// 2. Convert to virtual stack (VStack)

/*class FunctionSimple extends VStackComputableWithStep<String> {

  String color, shape;

  void step(VStack stack) {

    if (step == 0) {

      color = "black";

      step = 1;

    } else if (step == 1) {

      shape = "circle";

      step = 2;

    } else

      stack.return("A " + color + " " + shape);

  }

}

return vstackCompute(new FunctionSimple);*/

// 3. Add backtracking with BStack. (BacktrackableStack, BacktrackingStack, VStackWithBacktracking?)

global { sclass FunctionSimple extends VStackComputableWithStep<String> {

  String color, shape;

  void step(VStack stack) {

    cast stack to BStack;

    if (step == 0) {

      step = 1;

      stack.options(this,

        f -> f.color = "black",

        f -> f.color = "white");

    } else if (step == 1) {

      step = 2;

      stack.options(this,

        f -> f.shape = "circle",

        f -> f.shape = "square");

    } else

      stack.return("A " + color + " " + shape);

  }

}}

// Make a stack with our computation

BStack stack = new BStack(new FunctionSimple);

repeat 100 { // 100 steps max for safety

  // Get first or next result of function

  print("Result: " + stack.nextResultWithPrintStruct(10));

  // Ask the stack to backtrack to get another result

  stack = stack.backtrack();

  // If there is no more result, print message and exit

  if (stack == null) {

    print("No further result");

    return;

  }

  // There is another result, so continue loop

}