subtopic.clj

(def topic-fn [msg]
  ;; this is what I'm thinking about
)

(def p-ch (chan))
(def p1 topic-fn p-ch)

(def s-ch1
(sub p1 :foo s-ch1)

(def s-ch2
(sub p2 :foo.bar s-ch2) 
;;      ^-- the topic here is just an idea. any other representation of "nested topics" would be fine

(>!! p-ch {:topic :foo.baz}) ;; only s-ch1 would get this
(>!! p-ch {:topic :foo.bar}) ;; s-ch1 and s-ch2 would get this