more idiomatic clojure ..

index.clj

(let [mp (atom {})
      d [#{#{1 2}
           #{3 4}} ;node1
         #{#{5 6}
           #{7 8}}]] ;node2
  (dorun (for [nd d 
               nd-pair nd
               face nd-pair]
           (swap! mp update-in [face] #(conj % nd))))
  @mp)

; if you consider that points 1 2 3 4 form the node node1 and 5 6 7 8 form node2 
;I would like a map in the opposite direction ..  
;i.e. I should be able to find out all the nodes of which 1 is part of and 
;that is what the above code is doing.. but would like to know as to what 
;would be a more idiomatic way of doing this.