title: Simplest method to check whether unordered_map of unordered_maps contains key
body: <p>I am using an unordered_map of unordered_maps, such that I can reference an element using the "multi key" syntax:</p>
  
  <p><code>my_map[k1][k2]</code>. </p>
  
  <p>Is there a convenient way to use the same "multi-key" syntax to check whether an element exists before trying to access it? If not, what is the simplest way?</p>