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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 keys(self):
 """ return a list of the content types this set supports. this is not a complete list: serializers can accept more than one content type. However, it is a good representation of the class of content types supported. """

        return_value = []
        for s in self.serializers:
            return_value += s.content_type
        return return_value