Problem 89 instructions confuse me

(= 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?

Start with the Problem 89 instructions and the supplied edge-list expression; the payload names no source file or test. Determine how “unidirectional,” duplicate edges, and visiting each edge exactly once are intended to work, then revise the prompt or example so the expected false result and edge interpretation are unambiguous.