def trigger(cls, streams):
        """
        Given a list of streams, collect all the stream parameters into
        a dictionary and pass it to the union set of subscribers.

        Passing multiple streams at once to trigger can be useful when a
        subscriber may be set multiple times across streams but only
        needs to be called once.
        """
        # Union of stream contents
        items = [stream.contents.items() for stream in set(streams)]
        union = [kv for kvs in items for kv in kvs]
        klist = [k for k, _ in union]
        key_clashes = set([k for k in klist if klist.count(k) > 1])
        if key_clashes:
            clashes = []
            dicts = [dict(kvs) for kvs in items]
            for clash in key_clashes:
                values = set(d[clash] for d in dicts if clash in d)
                if len(values) > 1:
                    clashes.append((clash, values))
            if clashes:
                msg = ', '.join(['%r has values %r' % (k, v) for k, v in clashes])
                print('Parameter value clashes where %s' % msg)

        # Group subscribers by precedence while keeping the ordering
        # within each group
        subscriber_precedence = defaultdict(list)
        for stream in streams:
            stream._on_trigger()
            for precedence, subscriber in stream._subscribers:
                subscriber_precedence[precedence].append(subscriber)
        sorted_subscribers = sorted(subscriber_precedence.items(), key=lambda x: x[0])
        subscribers = util.unique_iterator([s for _, subscribers in sorted_subscribers
                                            for s in subscribers])

        with triggering_streams(streams):
            for subscriber in subscribers:
                subscriber(**dict(union))

        for stream in streams:
            with util.disable_constant(stream):
                if stream.transient:
                    stream.reset()