Since these are all single arg functions, I'm not sure if we have a huge win by switching. It makes more sense to me in [`unvisited-neighbors`](https://github.com/clojure-rica/maize/blob/83825ffa463bcab241e07a76f4b6da150b4a3d20/src-cljs/maze/core.cljs#L11) because we have a few fns with multiple args.

Happy to change it up though if it looks nicer:

```clojure
(defn- outer-walls [{:keys [size]}]
  (let [walls (for [x (range size) y (range size)]
                [#{[0 y] [-1 y]} #{[(dec size) y] [size y]}
                 #{[x 0] [x -1]} #{[x (dec size)] [x size]}])]
    (-> walls concat flatten set)))
```