title: Index field and blank out
body: <p>Is it possible to index a field and then blank it out?</p>
  
  <p>The reason for this would be that I have a plain text field and a field containing the encrypted version of the text. I'd like to index the plain text, and then remove it so only the encrypted data remains.</p>
  
  <p>I tried modifying the passed <code>doc</code> in my index function, but it doesn't seem to affect storage.</p>