title: Unsupported add/addAll operations for Map<K,V>.keySet()
body: <p>Regarding the <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Map.html?is-external=true" rel="nofollow noreferrer"><code>Map&lt;K,V&gt;</code></a> interface:</p>
  
  <p>Why does <code>keySet()</code> return a <code>Set</code> that supports the <code>remove</code> operation but doesn't support <code>add()</code> and <code>addAll()</code> operations?</p>