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

(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?))