def __init__(self, event_type,
                 subject=None,
                 objects=None,
                 context=None,
                 attributes=None,
                 timestamp=None,
                 scheduled=None):
        """
        :param EventType event: the type of event
        :param EntityRef subject: reference to the affected entity
        :param list objects: sequence of other affected objects (EntityRef)
        :param list context: sequence of affected systems (str)
        :param list attributes: sequence of affected attributes (str)
        :param datetime timestamp: when the event originated
        :param datetime schedule: when the event should be issued
        """
        self.event_type = event_type
        self.subject = subject
        self.timestamp = timestamp
        self.scheduled = scheduled
        self.objects = set(objects or [])
        self.context = set(context or [])
        self.attributes = set(attributes or [])