!7

cprint {
  class State extends ProbabilisticMachine2.State {
    IVF1<State> continuation;

    public State makeDescendant() {
      new State s;
      prepareDescendant(s);
      s.continuation = continuation;
      ret s;
    }
    
    void doneWithSubroutine {
      if (continuation == null) ret;
      IVF1<State> cont = continuation;
      addState(prepareDescendant(new State {
        run {
          cont.get(this);
        }
      }));
    }
    
    void step {
      doneWithSubroutine();
    }
  }
  
  start-thread {
    new ProbabilisticMachine2<State> pm;
    Chain<S> words = litchain("Hello", "World", ".");
    pm.addState(new State {
      printWords(words));
    });
    
    pm.onRunningState = st -> {
      print("Running state " + st);
    };
    pm.onRanState = st -> {
      print("Ran state " + st);
    };
    stepAllWithStats(pm);
  }
  
  State printWords(Chain<S> words) {
    if (empty(words)) ret new State;
    State s = prepareDescendant(new {
      run {
        print(first(words));
        addState(prepareDescendant(new State {
          run {
            dropFirst(words);
        
        doneWithSubroutine();
      }
    };
    s.continuation = state -> {
      print("World");
    };
    pm.addState(s);
  }
}

Began life as a copy of #1030671