gistfile1.clj

; given {1 [:a :b :c] 2 [:b :c :d]}
; flip the keys like so
;  {:a [1] :b [1 2] :c [1 2] :d [2]

(defn explode-keys-to-map                              
  "take [a b c] 1, return {a [1] b [1] c [1]}"         
  [m ks v]                                             
  (let [add-val-to-seq (fn [m k v]                     
                         (if (m k)                     
                           (assoc m k (conj (m k) v))  
                           (assoc m k [v])))]          
    (reduce #(add-val-to-seq %1 %2 v) m  ks)))         

(reduce (fn [m [k v]] (explode-keys-to-map m k v)) {} 
  (clojure.set/map-invert {1 [:a :b :c] 2 [:b :c :d]}))