The primary advantage of ungrammar is that it (eventually) allows one
to describe concrete syntax tree structure -- with alternatives and
specific sequence of tokens & nodes.
That should be re-usable for:
* generate `make` calls
* Rust reference
* Hypothetical parser's evented API
We loose doc comments for the time being unfortunately. I don't think
we should add support for doc comments to ungrammar -- they'll make
grammar file hard to read. We might supply docs as out-of band info,
or maybe just via a reference, but we'll think about that once things
are no longer in flux
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