title: Is it instance name/id that scala REPL prints?
body: <p><a href="https://docs.scala-lang.org/overviews/scala-book/set-class.html" rel="nofollow noreferrer">Tutorial</a> mentions about mutable sets in the initial but why would the REPL change the instance name from <code>res4</code> to <code>res5</code> when a new element is added? Is 'res' not the instance name that REPL prints? Below is the code in context. Beginner in scala. Please bear if the question is trivial. </p>
  
  <pre class="lang-scala prettyprint-override"><code>scala&gt; val set = scala.collection.mutable.Set[Int]()
  val set: scala.collection.mutable.Set[Int] = Set()
  
  scala&gt; set += 1
  val res0: scala.collection.mutable.Set[Int] = Set(1)
  
  scala&gt; set += 2 += 3
  val res1: scala.collection.mutable.Set[Int] = Set(1, 2, 3)
  </code></pre>