How do I group expressions in NHibernate?

Question

How can I group sets of expressions in NHibernate? For example, I want to filter my query like so:

(ShowOnDate IS NULL OR ShowOnDate <= GETDATE()) AND (ExpirationDate IS NULL OR ExpirationDate >= GETDATE())

I can add the 4 criterions separately, but I can't figure out how to emulate the paranthesis grouping. Thanks!

EDITED to show my final solution:

            result = this.Session.CreateCriteria<Model.News>()
                .Add(Expression.IsNull("ExpirationDate") || Expression.Gt("ExpirationDate", DateTime.Now.Date))
                .Add(Expression.IsNull("ShowOnDate") || Expression.Le("ShowOnDate", DateTime.Now.Date))
                .AddOrder(new Order("SubmittedDate", true))
                .List<Model.News>();

Answer 1

There is also Restrictions.And and Or for when you only need to combine two expressions...

criteria
    .Add(Restrictions.Or(Restrictions.IsNull("ShowOnDate"), Restrictions.Le("ShowOnDate", DateTime.Now)))
    .Add(Restrictions.Or(Restrictions.IsNull("ExpirationDate"), Restrictions.Ge("ExpirationDate", DateTime.Now)));