class TimelineState:
    def __init__(self):
        self.current_period = "modern"
        self.visited_periods = set(["modern"])
    
    def visit_period(self, period):
        self.current_period = period
        self.visited_periods.add(period)
    
    def get_current_period(self):
        return self.current_period
    
    def get_visited_periods(self):
        return list(self.visited_periods)