Counts number of bonus/stigma words.

        :param iterable words:
            Collection of words.
        :returns pair:
            Tuple with number of words (bonus words, stigma words).

def _count_words(self, words):
        """
        Counts number of bonus/stigma words.

        :param iterable words:
            Collection of words.
        :returns pair:
            Tuple with number of words (bonus words, stigma words).
        """
        bonus_words_count = 0
        stigma_words_count = 0

        for word in words:
            if word in self._bonus_words:
                bonus_words_count +=1
            if word in self._stigma_words:
                stigma_words_count += 1

        return bonus_words_count, stigma_words_count