:return:
        a seeder function that returns an event's source id only if that event's
        source space equals to ``event_space``.

def per_event_source_id(event_space):
    """
    :return:
        a seeder function that returns an event's source id only if that event's
        source space equals to ``event_space``.
    """
    def f(event):
        if is_event(event):
            v = peel(event)
            if v['source']['space'] == event_space:
                return v['source']['id']
            else:
                return None
        else:
            return None
    return _wrap_none(f)