title: How to set and get keys from scala TreeMap?
body: <p>Suppose I have</p>
  
  <pre><code>import scala.collection.immutable.TreeMap
  
  val tree = new TreeMap[String, List[String]]
  </code></pre>
  
  <p>Now after above declaration, I want to assign key "k1" to List("foo", "bar")
  and then how do i get or read back the key "k1" and also read back non-existent key "k2"?</p>
  
  <p>what happens if I try to read non-existent key "k2" ?</p>