def get_last_visit(user_summary: Dict[(str, Dict[(str, datetime)])], k: str) -> Optional[datetime]:
    if (k in user_summary):
        return user_summary[k]['last_visit']
    else:
        return None
def get_last_visit(user_summary: Dict[(str, Dict[(str, datetime)])], k: str) -> Optional[datetime]:
    if (k in user_summary):
        return user_summary[k]['last_visit']
    else:
        return None