Mutually recursive specs in clojure.spec

oddeven.clj

;;; Clojure 1.9 spec : an example of a mutually-recursive spec
(ns oddeven
  (:require [clojure.spec :as s]))

(s/def ::zero #{:zero})
(s/def ::one #{:one})
(s/def ::succ #{:succ})

(s/def ::odd nil)
(s/def ::even (s/or :zero ::zero
                    :even (s/tuple ::succ ::odd)))
(s/def ::odd (s/tuple ::succ ::even))

(s/conform ::even :zero) ; zero is even
;; => [:zero :zero]

(s/conform ::even [:succ [:succ :zero]])  ; 2 is even
;; => [:even [:succ [:succ [:zero :zero]]]]

(s/conform ::odd [:succ :zero]) ; 1 is odd
;; => [:succ [:zero :zero]]

(s/conform ::odd [:succ [:succ :zero]]) ; 2 is not odd
;; => :clojure.spec/invalid
(s/explain ::odd [:succ [:succ :zero]]) ; (explanations...)
;; In: [1] val: [:succ :zero] fails spec: :typetheory.syntax/zero at: [1 :zero] predicate: #{:zero}
;; In: [1 1] val: :zero fails spec: :typetheory.syntax/odd at: [1 :even 1] predicate: vector?
;; => nil

(s/exercise ::even)
;; => generates  10 even numbers