define-language

Sample usage:

(define-language (process-expr define-expr define-expr-clause))

(define-expr (sym symbol)
  (print sym))

(define-expr-clause neato (&optional thing)
  (format t "~&neato! ~a~%" thing)
  (parse-expr thing))

;; (parse-expr 'foo) ; prints and returns `FOO'

;; (parse-expr '(neato foo)) ; prints `neato! FOO', then prints and returns `FOO'