title: Convert Map to ImmutableSetMultimap
body: <p>I am trying to convert the following piece of code to use <code>ImmutableSetMultimap</code>, but I run into problems when I try to do <code>map.get(...).add(...)</code> because its immutable.  Is there an easy way to do this?</p>
  
  <pre><code>List&lt;MyObject&gt; objects   
  
  Map&lt;Long, Set&lt;Key&gt;&gt; map = new HashMap&lt;Long, Set&lt;Key&gt;&gt;();
  for (MyObject entry : objects) {
      if (map.containsKey(entry.getId())) {
          map.get(entry.getId()).add(entry.getKey());
      } else {
          Set&lt;Key&gt; newSet = new HashSet&lt;Key&gt;();
          newSet.add(entry.getKey());
          map.put(entry.getId(), newSet);
      }
  }
  </code></pre>