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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 _get_contrast(self, con_id):
 """ Retreive contrast with identifier 'con_id' from the list of contrast objects stored in self.contrasts """

        for contrasts in list(self.contrasts.values()):
            for contrast in contrasts:
                if contrast.estimation.id == con_id:
                    return contrast
        raise Exception("Contrast activity with id: " + str(con_id) +
                        " not found.")