!7

cprint {
  class State extends ProbabilisticMachine2.State {
    ImmutableListWalker<S> inputWalker;
    Chain stack;
    
    public State makeDescendant() {
      State s = prepareDescendant(new State);
      s.inputWalker = inputWalker;
      s.stack = stack;
      ret s;
    }
    
    void doneWithSubroutine {
      if (stack == null) ret;
      State s = makeDescendant();
      s.stack = dropFirst(s.stack);
      process(s, first(stack));
    }
    
    void step {
      doneWithSubroutine();
    }
  }
  
  record SequenceCons(a, b) > State {
    void step {
      State s = makeDescendant();
      s.stack = chainPlus(s.stack, b);
      process(s, a);
    }
  }
    
  record grab(S what) > Executable {
    void step(State state) {
      if (state.inputWalker.atEnd()) ret;
      S token = state.inputWalker!;
      if (eq(token, what)) {
        State s = state.makeDescendant();
        s.inputWalker = s.inputWalker.next();
      }
    }
  }
  
  class EndOfInput > Executable {
    void step(State state) {
      print("end of input reached");
      super.step();
    }
  } // what: string or a GrammarClass
  
  Executable sequence(O... steps) {
    if (empty(steps)) ret new Executable;
    ret foldl_noSeed((a, b) -> new SequenceCons(a, b), steps);
  }

  start-thread {
    Sequence main = sequence(
      new grab("Hello"),
      new grab("world"),
      new EndOfInput);

    printIndentedStruct(main);
    
    S input = "Hello world";
    new ProbabilisticMachine2 pm;
    pm.withState(new State {
      {
        inputWalker = new ImmutableListWalker(javaTokC(input));
      }
    }, r {
      main.step(pm.currentState!);
    });

    stepAllWithStats(pm);
  }
}

Began life as a copy of #1030667