sclass SimpleInterpreter {
  sclass State {
    new LinkedHashMap<S, O> vars;
    new L stack;
    
    selfType set(S var, O value) {
      vars.put(var, value);
      this;
    }
    
    O eval(Instruction i) {
      i.run(this);
      ret pop();
    }
    
    O get(S var) { ret mapGet(vars, var); }
    
    void push(O value) { stack.add(value); }
    O pop() { ret popLast(stack); }
  }
  
  asclass Instruction {
    abstract void run(State state);
  }
  
  srecord GetVar(S var) > Instruction {
    void run(State state) {
      state.push(state.get(var));
    }
  }
  
  srecord Sequence(L<Instruction> l) > Instruction {
    Sequence(Instruction... l) { this.l = asList(l); }
    
    void run(State state) {
      fOr ping (Instruction i : l)
        i.run(state);
    }
  }
  
  State newState() { ret new State; }
}