(s/def ::obj-type #{:a :b})
(s/def ::attribute int?)

(s/def ::base-obj (s/keys :req-un [::obj-type ::attribute]))

(s/def ::only-in-a #{"a"})
(s/def ::only-in-b #{"b"})
(defmulti obj-type :obj-type)
(defmethod obj-type :a [_]
  (s/merge ::base-obj (s/keys :req-un [::only-in-a])))
(defmethod obj-type :b [_]
  (s/merge ::base-obj (s/keys :req-un [::only-in-b])))

(s/def ::obj (s/multi-spec obj-type :obj-type))

(s/exercise ::obj 20 {::obj-type #(gen/return :a)})
; returns maps with both ":a" and ":b"
(s/exercise ::base-obj 20 {::obj-type #(gen/return :a)})
; returns maps with only ":a"
; why does the generator work in the second example?