- put the tests together with the other define-union-language tests
- fix the docs
- add a release note
- construct the merged language a bit more directly
- properly deal with this kind of thing:
(define-language L1 (e f ::= 1 2 3))
(define-language L2 (e g ::= 4 5 6))
(define-union-language L L1 L2)