untitled.clj

(defn mystr [x y] (str x ", " y))

(defparallelagg myagg :init-var #'identity :combine-var #'mystr)

(let [tap [[:a 1]
           [:a 2]
           [:b 3]
           [:c 4]]]
  (?<- (stdout) [?label ?nums]
       (tap ?label ?all-nums)
       (myagg ?all-nums :> ?nums)))