@Override
  @Nullable
  public Object computeExpression(PsiElement expression,
                                  boolean throwExceptionOnOverflow,
                                  @Nullable PsiConstantEvaluationHelper.AuxEvaluator auxEvaluator) {
    if (!(expression instanceof GrExpression)) return null;
    return evaluate((GrExpression)expression);
  }
@Override
  @Nullable
  public Object computeExpression(PsiElement expression,
                                  boolean throwExceptionOnOverflow,
                                  @Nullable PsiConstantEvaluationHelper.AuxEvaluator auxEvaluator)