title: Scala prepend +: op not modifying the ListBufffer
body: <p>I am using the prepend method on the listbuffer type and observing some weird behavior. The prepend operation returns a new list which is acceptable . But shouldn't it also modify the ListBuffer ? After prepending I am still seeing that the length of the ListBuffer is not changed. Am I missing something here?</p>
  
  <pre><code>scala&gt; val buf = new ListBuffer[Int]
  buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer()
  
  scala&gt; buf += 1                 
  res47: buf.type = ListBuffer(1)
  
  scala&gt;  buf += 2
  res48: buf.type = ListBuffer(1, 2)
  
  scala&gt; 3 +: buf
  res49: scala.collection.mutable.ListBuffer[Int] = ListBuffer(3, 1, 2)
  
  scala&gt; buf.toList
  res50: List[Int] = List(1, 2)
  </code></pre>