*/ 
  public String error_string () {
    switch (error ()) {
    case NO_ERROR: return "no-error";
    case INVALID_ENUM: return "invalid-enum";
    case INVALID_VALUE: return "invalid-value";
    case INVALID_OPERATION: return "invalid-operation";
    case STACK_OVERFLOW: return "stack-overflow";
    case STACK_UNDERFLOW: return "stack-underflow";
    case OUT_OF_MEMORY: return "out-of-memory";
    default: return null;
    }
  }