<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _generate_phrases(self, sentences):
 """Method to generate contender phrases given the sentences of the text document. :param sentences: List of strings where each string represents a sentence which forms the text. :return: Set of string tuples where each tuple is a collection of words forming a contender phrase. """

        phrase_list = set()
        # Create contender phrases from sentences.
        for sentence in sentences:
            word_list = [word.lower() for word in wordpunct_tokenize(sentence)]
            phrase_list.update(self._get_phrase_list_from_words(word_list))
        return phrase_list