cat

functionsince v0.0-2341 clojure.core/catEdit
(cat rf)

Source docstring:
A transducer which concatenates the contents of each input, which must be a
collection, into the reduction.
Source code @ clojurescript:src/main/cljs/cljs/core.cljs
(defn cat
  [rf]
  (let [rf1 (preserving-reduced rf)]
    (fn
      ([] (rf))
      ([result] (rf result))
      ([result input]
         (reduce rf1 result input)))))