Generate next character sampled from the distribution of characters next.

def _gen_next(self, history):
        """Generate next character sampled from the distribution of characters next.
        """
        orig_history = history
        if not history:
            return helper.START
        history = history[-(self._n-1):]
        kv = [(k, v) for k, v in self._T.items(history)
              if k not in reserved_words]
        total = sum(v for k, v in kv)
        while total == 0 and len(history) > 0:
            history = history[1:]
            kv = [(k, v) for k, v in self._T.items(history) 
                  if k not in reserved_words]
            total = sum(v for k, v in kv)
        assert total > 0, "Sorry there is no n-gram with {!r}".format(orig_history)
        _, sampled_k = list(helper.sample_following_dist(kv, 1, total))[0]
        # print(">>>", repr(sampled_k), len(history))
        return sampled_k[len(history)]