title: Code does not respond to change in attribute
body: <p>So I have <code>aNum</code> as a patches-own attribute, and in my code I have 
  <code>ask patch 0 -5 
  [set aNum (aNum - 1)]</code> but the patch just has changes its aNum to -1. </p>
  
  <p>How can I make sure it takes what aNum it has been previously assigned and subtracts 1 from that instead?</p>
  
  <p>Thank you!</p>