J-style "under" in Plan. Might go in the standard library. (cf. http://prog21.dadgum.com/121.html)

gistfile1.txt

; don't use "inverse" because it's not the same as a mathematical inverse
; and maybe we'll have a separate system for keeping track of those later.
; also possibly set-opposite should be called set-opposite!

(let opposites+ {}
  (deffn (opposite f) (opposites+ f))
  (deffn (set-opposite f g)
    (set (opposites+ f) g)))

(defmac (under var (f &s) &body)
  (let ,var (,f ,@s)
    (protect
      ,@body
      ((opposite ,f) ,var))))

(set (setter opposite) set-opposite)
(set (opposite open) close)
(set (opposite socket) close)
(set (opposite socketpair) (partial map close))

As of writing, this won't work in the Plan in the git repo, because it requires function arguments to be destructuring, which they're not there yet. It also uses the hypothetical "protect" statement, which, along with the rest of the error-handling system, is not implemented currently.