title: bounce turtle from green patches in netlogo world
body: <p>I'm trying to implement a bounce turtle logic. When any turtle move towards green patches, I want to set <code>turtles head = set head ( - head)</code></p>
  
  <p>My code is:</p>
  
  <pre><code>ask turtles [ask turtles [
    ifelse [pcolor] of patch-ahead 2 !=[move]  
  [ set heading ( - heading) ]
  </code></pre>
  
  <p>Question: but it gives error ( OF expected input to be a turtle agentset or patch agentset or turtle or patch but got NOBODY instead).
  How to solve this error?</p>