https://stackoverflow.com/questions/20136617/clojure-idiomatic-update-a-maps-value-if-the-key-exists

update-in-if-contains.clj

(defn contains-in?
  [m ks]
  (not= ::absent (get-in m ks ::absent)))

(defn update-in-if-contains
  [m ks f & args]
  (if (contains-in? m ks)
    (apply (partial update-in m ks f) args)
    m))

;; (update-in-if-contains map [:key] map-fn)

(defn helpme [m]
  (cond-> m
    (:r m) (assoc :r [])
    (:g m) (assoc :g [])))

;; will only replace :r if :r exists, concise way to do option.map