title: Select an expression with nhibernate queryover
body: <p>using nHibernate QueryOver I want to select a series of values that include an aggregate expression. So the sql might be:</p>
  
  <pre><code>SELECT SUM(Total / (TaxRate + 1)) Totals FROM Contract Group By CustomerId
  </code></pre>
  
  <p>I can't see where (or if) this is supported. It seems odd that I can write where clauses as expressions but not define columns in the select as expressions.</p>