title: What values should I use for iOS boolean states?
body: <p>It appears that in iOS I have a number of options that seem to fit for boolean values:</p>
  
  <pre><code>YES
  NO
  TRUE
  FALSE
  true
  false
  </code></pre>
  
  <p>Which ones should I use? In this particular case I'm hiding a label, so should I set the <code>hidden</code> property to <code>YES</code>, <code>TRUE</code>, or <code>true</code>?</p>