Played around with it a bit more, r6rs appears problematic as apparently ++ is not a legal symbol in r6rs (!)
I took a look at PLT's implementation of lambda for r6rs/r5rs (it's in collects/r5rs/main.ss), and they just simply convert to a mutable list if the lambda has a rest parameter:
(define-syntax (r5rs:lambda stx)
;; Convert rest-arg list to mlist, and use r5rs:body:
(syntax-case stx ()
[(_ (id ...) . body)
(syntax/loc stx (#%plain-lambda (id ...) (r5rs:body . body)))]
[(_ (id ... . rest) . body)
(syntax/loc stx
(#%plain-lambda (id ... . rest)
(let ([rest (list->mlist rest)])
(r5rs:body . body))))]))
(the list->mlist is the part I'm looking at)
So having for example (fn args ...) compile to an (arc-lambda args ...) which does the same thing might be simpler than trying to get Arc to compile and run in the whole complicated r6rs environment.
