title: persistent: How do I get the I in ACID
body: <p>Assume I'm doing the following steps in a transaction:</p>
  
  <ul>
  <li>read some data A from the database</li>
  <li>do some calculation based on it</li>
  <li>write some data B to the database</li>
  </ul>
  
  <p>Is it possible to make this transaction fail if A has changed in the mean time?</p>
  
  <p>In short: How do I achieve isolation in the persistent package?</p>