(foldcat coll)
Equivalent to (fold cat append! coll)
;; Worth remembering that parallel fold in Clojure is enabled for:
;; vectors, maps and clojure.core.reducers.Cat objects.
;; r/foldcat returns a "Cat", so it can be parallel folded again:
(r/fold +
(r/foldcat
(r/filter even?
(r/foldcat
(r/map inc (into [] (range 100000)))))))
;; 2500050000