Lisp challenge

Lisp macros can do anything, they say. But can they do typeclasses?
For the purpose of feasibility study, I'll simplify the challenge, of course. It will go like this:

Implement the form mappend such that

(mappend 2 3)

expands to

(+ 2 3)

and

(mappend "arst" "neio")

expands to

(concatenate 'string "arst" "neio")

(the above code is in Common Lisp, but you're free to use any Lisp and substitute the corresponding forms from that Lisp).

The expansion must happen no later than compile-time, with no runtime overhead.

Feel free to use Typed Racket, Shen, Arc, Clojure or any other Lisp.

>>11
Yeah, you deluded yourself so far that you think >>8's code looks pretty or elegant.