it's used in `append_pyout`, it's the third arg to `append_mime_type`.  I have a detailed note about it in the PR discussion (a couple posts above), but basically it just exists to add an `output_pyout` class.  Which I'm +0 on, it's nice because we have an `output_pyerr` class, but it sucks because I had to add the `extra_class` thing.