arsspeak

(ns useless-set-theoretic-numbers
  "How do you arrive at numbers instead of starting with them?"
  (:require [clojure.set]))

(def zero #{})
(defn successor [x] (conj x x))
(defn predecessor [x]
  (if-not (= x zero)
    (apply clojure.set/union x)
    (throw (ex-info "Can't go below zero" {}))))