# new new new
        years = sorted((variant.intro_date, variant.end_date) for variant in self.model_variants)

	# new new
        years = set()
        for variant in self.model_variants:
            years.add((variant.intro_date, variant.end_date))
        years = sorted(years)

	#new
        years = set()
        for dates in [(variant.intro_date, variant.end_date) for variant in self.model_variants]:
            years = years.union(dates)
        years = sorted(years)

        # old
        years = sorted(reduce(set.union, [(variant.intro_date, variant.end_date) for variant in self.model_variants], set()))