title: Python dictionary adding new values in existing key
body: <pre><code>def add_to_dict(word_to_nexts, word, next_word):
      ''' (dict of {str: set of str}, str, str) -&gt; NoneType
  
      Update word_to_nexts so that word maps to a set containing
      next_word
  
      &gt;&gt;&gt; my_dict = {'one':{'day', 'two'}, 'a':{'dog', 'day'}}
      &gt;&gt;&gt; add_to_dict(my_dict, 'one', 'time')
      &gt;&gt;&gt; my_dict == {'one':{'day', 'two', 'time'}, 'a':{'dog', 'day'}}
      True
  
      '''
  </code></pre>
  <p>I wonder, is there any built-in method that I can use for finishing this task?
  I was trying to do</p>
  <pre><code>    for i in word_to_nexts:
          if i == word:
              word_to_nexts[i].add(next_word)
  </code></pre>
  <p>however, I did not get the boolean value I expected.</p>