(
 (fn tour?
   ([g] (some boolean (map (partial tour? g) (set (flatten g))))) ; try each node as starting point
   ([g r] ; is the graph visitable from the "r" root node?
   	(let [connected? (fn [n [x y]] (cond (= n x) y (= n y) y)) ; return the connected node, if any
          connected (keep-indexed ; return the nodes connected to the root, and their index
                     #(let [conn (connected? r %2)]
                        (if conn [conn %1])) g)
          candidates (map (fn [[n i]] [(concat (take (dec i) g) (drop i g)) n]) connected)] ; candidates are "new" root nodes, and the remaining graphs
         (or (empty? g)
             (and (not (empty? connected))
                  (some boolean (map #(apply tour? %) candidates)))))))
[[:a :b] [:a :c] [:c :b] [:a :e]
              [:b :e] [:a :d] [:b :d] [:c :e]
              [:d :e] [:c :f] [:d :f]])