(define goal?
   S -> (= (sum S) 27))

(define f
   S -> (remove-if-no-good  [[2 | S] [7 | S] [9 | S]]))

(define remove-if-no-good
  Ss -> (remove-if* (/. X (> (sum X) 27)) Ss))

(define remove-if*
  _ [] -> []
  F [X | Y] -> (remove-if F Y)	where (F X)
  F [X | Y] -> [X | (remove-if F Y)])

(define return-n
   10 <- (fail)
   10 -> 9
    N -> N)

(define return-n
  N <- (fail-if (/. X (> X 45)) N))

(define depth
  State F G?  -> (depth-help [State] F G?))

(define depth-help
        [State | _] _ G? -> State 		where (G? State)
        [State | _] F G? <- (depth-help (F State) F G?)
        [_ | States] F G? -> (depth-help States F G? )
        _ _ _ -> (fail))

(depth [] (fn f) (fn goal?))