scratch.clj

(def graph {:a [:b :c :g]
            :b [:e :g]
            :c []
            :d []
            :e [:a]
            :f [:c :d]
            :g [:e]})

(defn find-paths [graph from to]
  (let [children (fn [[path visited node]]
                   (->> node
                     graph
                     (remove visited)
                     (map #(vector (conj path %)
                                   (conj visited %)
                                   %))))]
    (->> ((juxt vector hash-set identity) from)
      (tree-seq children children)
      (filter (comp #{to} peek))
      (map first))))

(comment
  user> (find-paths graph :a :e)
  ([:a :b :e] [:a :b :g :e] [:a :g :e])

  user> (find-paths graph :g :d)
  ()

  user> (find-paths graph :g :a)
  ([:g :e :a]))