TODO

- Make external default (fn.zip) + add --inline option
- Recognize `if __name__ == '__main__':` and allow its contents to process like
  separate cells. (Need to be able to skip the other branches of the
  conditional.)
- Clean up evaluation:
    - Get rid of eval_expression (exec_eval already does the job)
    - Capture errors correctly
        - Save traceback in REPL
        - This is mostly needed in REPL. In normal run, we should abort on error.
        - [Done] Preserve checkpoints, if we catch an exception. In general,
          reconcile whether the notebook (the processed part) should be written
          out, if an exception occurs.
        - [Done] Exceptions thrown during checkpointing shouldn't crash the
          system, just report that checkpointing failed. (E.g., if an object is
          not pickleable.)
    - [No reason] Lift more logic from ptpython
    - [Fixed: locals = globals] Possibly related: debug usage with joblib.Parallel
- Stream cells (via a generator) to produce output faster
- Better, explicit Vim support
- Add option to embed both KaTeX and water.css into HTML output (to have a
  standalone page).

Done:

- Add breakpoint cell that drops into repl (and then resumes execution)
- Checkpointing is broken in the interactive mode.
- Add ability to save assets in an external zip file.
- Add padding logic to better preserve blank lines around cells.
- Add --html option to convert.
- Add convert command for converting and showing Jupyter notebooks.
- Maybe change formatting of the cell prefixes (#m>, #o>, #chk>)
- Investigate checkpoint size.
    - Nothing wrong here. Perhaps, it's worth compressing checkpoints.
- Write README
- Clean up parser
- Add more cell types:
    - [Done] variable memoizing code cells (specify which variables this cell produces,
      caching them and skipping computation on subsequent evaluation)
    - [Done] non-skippable cells (i.e., even if resuming from a checkpoint, these cells
      will execute, before the checkpoint is loaded)
- Capture matplotlib better (e.g., multiple figures in a loop)
- ptpython
    - history in terms of the code cells
    - clean up various warnings (e.g., the message about meta+enter)
- MPI support: if running under MPI only rank 0 should show output and save the
  notebook
- Clean up output:
    - [Done] flush lines as they are being output (test with progress bars and such)
    - [Done] check that output from C extensions is captured
    - [Done] capture and properly output stderr
    - [Fixed: rich-text parsing] debug the list output problem
- Add tests
- Add our own icat implementation
- icat output is being captured, but not if redirected to a file
- Add images command to extract PNGs out of the notebook
- Better support for ANSI codes (e.g., to handle two-line tqdm)
- Handle sys.exit
- Clean up traceback (line numbers are broken across cells)
- [Done: not a bug] fig.clear() and/or ax.clear() with Matplotlib (many cheatsheets are failing)
    - the problem was with using plt.show(), rather than fig.show()