title: Quick Explanation of SUBQUERY in NSPredicate Expression
body: <p>There appears to be zero documentation about the SUBQUERY keyword from Apple and I can't find a simple explanation about it on SO or on Google. It's a conspiracy! ;)</p>
  
  <p><em>Please</em>, could someone from the inner-circle please just provide a quick explanation of its syntax so I can use it?</p>
  
  <pre><code>SUBQUERY(Bs, $x, $x IN %@)
  </code></pre>
  
  <p>Thanks</p>