apply

; find-all-to-all returns a lazy list of lists of object instances
; and we want a map of instance vs. number of appearances


; This one blows up RAM:

(apply merge-with +
  (map #(zipmap % (repeat 1))
    (find-all-to-all-paths nodes degrees)))

; This one doesn't:

(reduce
  #(reduce
    (fn [m ^Node nd] (assoc m nd (inc (get m nd 0))))
    %1 ; a map
    %2) ; a vector of nodes
  {}
  (find-all-to-all-paths nodes degrees))