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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 sort_by(self, sf):
 """ Determines how to sort search results. Available sorting methods are sort.SCORE, sort.COMMENTS, sort.HEIGHT, sort.RELEVANCE, sort.CREATED_AT, and sort.RANDOM; default is sort.CREATED_AT. """

    params = join_params(self.parameters, {"sf": sf})

    return self.__class__(**params)