title: Kotlin: "if item not in list" proper syntax
body: <p>Given Kotlin's list lookup syntax,</p>
  
  <pre><code>if (x in myList)
  </code></pre>
  
  <p>as opposed to idiomatic Java, </p>
  
  <pre><code>if (myList.contains(x))
  </code></pre>
  
  <p>how can one express negation?  The compiler doesn't like either of these:</p>
  
  <pre><code>if (x not in mylist)
  
  if !(x in mylist)
  </code></pre>
  
  <p>Is there an idiomatic way to express this other than <code>if !(mylist.contains(x)))</code>?  I didn't see it mentioned in the <a href="https://kotlinlang.org/docs/reference/control-flow.html" rel="noreferrer">Kotlin Control Flow docs.</a></p>