(= false (__ [[:a :b] [:a :b] [:a :c] [:c :a]
               [:a :d] [:b :d] [:c :d]]))

So, I am not actually sure what to make of this. I interpreted "unidirectional" to mean that [:a :c] and [:c :a] were the same edge. But if I disregard the two duplicate edges, it seems to me that (d c) (c a) (a d) (d b) (b a) is a valid solution to this. Is the intent that this one is necessarily false because some edges appear twice in the list, and you can't visit them all while only visiting them exactly once, or am I missing something else?