code: def collect_video_streams(self):
          
          rc = []
          streams_by_id = {}
  
          # scan first for all streams
          for t in self.all_tags_of_type(TagDefineVideoStream):
              stream = [ t ]
              streams_by_id[t.characterId] = stream
              rc.append(stream)
  
          # then find the frames
          for t in self.all_tags_of_type(TagVideoFrame):
              # we have a frame for the /named/ stream
              assert t.streamId in streams_by_id
              streams_by_id[t.streamId].append(t)
  
          return rc
docs: Return a list of video streams in this timeline and its children.
          The streams are returned in order with respect to the timeline.
  
          A stream is returned as a list: the first element is the tag
          which introduced that stream; other elements are the tags
          which made up the stream body (if any).