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I've looked at the github project for Mathics and didn't see any use of ply... Looks like they have written their own parser/lexer.

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Yes, indeed - it is very weird! Because mathics still mentions ply in change.rst, but not used in code anymore:

https://github.com/mathics/Mathics/blob/master/CHANGES.rst

Hidden in there (under 1.0) is "new recursive descent parser #416". We've only just finished 1.0 and haven't had a chance to announce it properly yet.

is this limitation in ply?

We previously used ply but ran into a few issues:

The main issue was that generating the tables was extremely slow (~15mins). We solved this by packaging the tables, but when the version of ply changed a table regeneration was forced on our users.

The recursive descent parser was also simpler and significantly more performant.

There's a short write-up on the PR: https://github.com/mathics/Mathics/pull/416

Mathics developer here. Yes, we rewrote it. I better update the docs.

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