Things to do for syntax/parse
=============================

TEST & DOC - Generalize stxclass arities.
TEST & DOC - provide-syntax-class/contract

Refine expr/c.

Wrap default args in stxclass parameters with 'this-syntax'
  - other stxparams? like 'this-base-syntax' etc?

Add debugging mode that records *all* intermediate patterns 
on expectstack.

Add "roles" to error messages, eg
  expected identifier for foo thingummy name
instead of the current
  expected identifier

Improve ~do.

Improve reflection.

More cowbell.

Reorganize tests.

Allow reflected syntax classes in conventions.

Rename "conventions" to "convention-set"?

For documentation, talk about "primary attributes" vs "nested
attributes". Helps explain ~eh-var and #:auto-nested-attributes.

Fix syntaxes pinpointed for repetition constraint violations.