code: def per_event_source_id(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)
docs: :return:
          a seeder function that returns an event's source id only if that event's
          source space equals to ``event_space``.