code: def _get(self, word1, word2):
          
          key = self._WSEP.join([self._sanitize(word1), self._sanitize(word2)])
          key = key.lower()
          if key not in self._db:
              return
  
          return sample(self._db[key], 1)[0]
docs: Return a possible next word after ``word1`` and ``word2``, or ``None``
          if there's no possibility.