title: Scala: Iterate over mutable map of maps
body: <p>I have defined a mutable map of maps</p>
  
  <pre><code>import scala.collection.mutable.Map
  
  val default = Map.empty[String, Int].withDefaultValue(0)
  val count = Map.empty[Any, Map[String, Int]].withDefaultValue(default)
  </code></pre>
  
  <p>which I populate/update as in</p>
  
  <pre><code>count("furniture")("table") += 1
  count("furniture")("chair") = 6
  count("appliance")("dishwasher") = 1
  </code></pre>
  
  <p>How can I iterate over all items in <code>count</code>? And why does <code>count.keys</code> return an empty <code>Set()</code>?</p>