title: Check if key exists in dictionary of type [Type:Type?]
body: <p>How can I check if a key exists in a dictionary? My dictionary is of type <code>[Type:Type?]</code>.</p>
  
  <p>I can't simply check <code>dictionary[key] == nil</code>, as that could result from the value being <code>nil</code>.</p>
  
  <p>Any ideas?</p>