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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 refine (self, requirements):
 """ Refines this set's properties using the requirements passed as an argument. """

        assert isinstance(requirements, PropertySet)
        if requirements not in self.refined_:
            r = property.refine(self.all_, requirements.all_)

            self.refined_[requirements] = create(r)

        return self.refined_[requirements]