(require '[clojure.spec :as s])

(s/fdef clojure.core/subs
        ;; the args are a string, integer and optionally another integer
        :args (s/cat :s string? :start integer? :end (s/? integer?))
        ;; the return value should be a string
        :ret string?
        ;; the input string should include the returned string
        :fn #(clojure.string/includes? (-> % :args :s) (:ret %)))

(s/instrument #'clojure.core/subs)

(subs 1 2 3) ;; =>

;; ExceptionInfo Call to #'clojure.core/subs did not conform to spec:
;; In: [0] val: 1 fails at: [:args :s] predicate: string?
;; :clojure.spec/args  (1 2 3)
;; clojure.core/ex-info (core.clj:4617)

(subs "foo" 1 2) ;; => "o", satisfies :ret and :fn obviously