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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def subtrees_for_phrase(self, phrase_type):
 """ Returns subtrees corresponding all phrases matching a given phrase type :param phrase_type: POS such as "NP", "VP", "det", etc. :type phrase_type: str :return: a list of NLTK.Tree.Subtree instances :rtype: list of NLTK.Tree.Subtree """

        return [subtree for subtree in self.parse.subtrees() if subtree.node.lower() == phrase_type.lower()]