docs: def scene_velocity(frames):
          """parse a scene.velocity message"""
          # "scene.velocity" <scene_id> <velocity>
          reader = MessageReader(frames)
          results = reader.string("command").uint32("scene_id").uint32("velocity").assert_end().get()
          if results.command != "scene.velocity":
              raise MessageParserError("Command is not 'scene.velocity'")
          return (results.scene_id, results.velocity/1000)