!747

abstract class Pred {
  boolean eval(Map<S, S> s);
}

m {
  static int nextMCellID;
  static new Map<S, MCell> memoryCells;
  
  // a memory cell
  static class MCell {
    S id;
    O value;
    long lastAccess;
    
    *(S *id) {
      lastAccess = now();
    }
    
    O get() {
      lastAccess = now();
      return value;
    }
    
    void set(O value) {
      lastAccess = now();
      this.value = value;
    }
  }
  
  static MCell newMCell() {
    S id = "m" + ++nextMCellID;
    MCell cell = new MCell(id);
    memoryCells.put(id, cell);
    return cell;
  }
  
  static MCell getMCell(S id) {
    return memoryCells.get(id);
  }
  
  p {
    defineVerification("<memory cell> is occupied.",
      new Pred() {
        boolean eval(Map<S, S> s) {
          S cell = s.get("memory cell");
          return memoryCells.get(cell).get() != null;
        }
      });
    verify("m1 is occupied.");
  }
  
  static void defineVerification(S sentence, Pred p) {
  }
}