Can someone help explain to me how walker works. Below I'm trying to exclude all maps and submaps that contain a value "red"

(defn has-red? [x]
  (prn x)
  (cond (map? x) (nil? ((-> x vals set) "red"))
        :else false))

(let [j [{"a" 2
          "b" 4
          "d" "green"
          "e" [{"z" "zzz"
                "y" "yyy"}]
          "c" {"d" 5
               "e" "red"
               "g" {"h" 9
                    "i" \a}}
          "m" {"aa" 99}}
         {"sdfsdf" "gsdgdfgdfg"
          "fsf" 979
          "d" "orange"
          "e" [{"z" "red"
                "y" "yyy"}
               {"jfoj" "purple"
                "fffj" 5}]
          "c" {"d" 5
               "e" "purple"
               "g" {"h" 9
                    "i" \a}}
          "m" {"red" 99}}]]
  (select (walker has-red?) j))