- In anticipation of megaparsec 7, which removes support for stacks of
include files (as far as I can tell)
- Intended for the 'StateT Journal' layer of the parser
- A stack of include files would be better in a 'ReaderT' layer, but I
don't want to add another layer to the parser
- Intended for detecting cycles of include files
- Potential issue: for proper error messages for include file cycles,
we must remember to provide the filepath of the root journal file via
the initial journal state passed to a 'JournalParser'; I imagine
that we may forget to do so because in all other cases it is okay
not to do so.
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