title: What does ?: do in Kotlin? (Elvis Operator)
body: <p>I can't figure out what <code>?:</code> does in for example this case </p>
  
  <pre><code>val list = mutableList ?: mutableListOf() 
  </code></pre>
  
  <p>and why can it be modified to this </p>
  
  <pre><code>val list = if (mutableList != null) mutableList else mutableListOf()
  </code></pre>