We have a way to extend languages, but not a way to extent passes

[akeep]

We would like to have a way to create a "template" language with a "template" pass, and then extend both the language and the pass to create a concrete pass.

We would like to be able to extend both the existing transformers and the list of definitions, expressed using something like the - and + syntax that we extend passes with.

The idea is to have something like:

(define-pass foo-template : L-template (x) -> L-template ()
  (definitions
     (define f1 (lambda (x) ---))
     (define f2 (lambda (y) ---))
    ---)
  (A : A (x) -> A ()
    [(a) `(a)]
    [(b ,x ,y ,z) `(b ,x ,y ,z)]
    ---)
  (B : B (x) -> B ()
    [(qux ,x) `(qux ,x)]
    ---)
  body)

and modify it as (syntax subject to change):

(define foo : (L (extends L-template)) (x) -> (L (extends L-template)) ()
  (extends foo-template)
  (definitions
    (- (define f2 ...))
    (+ (define f3 (lambda (z) ---)))
  (A : A (x) -> A ()
    (- [(b ,x ,y ,z) ...])
    (+ [(c ,w) `(c ,w)]
        [(d ,n) `(d ,n)]))
  (+ (C : C (x) -> C ()
        ---)
  (- body)
  body)