@carkh

(s/conform
  (s/cat :tag keyword? :children (s/& (s/+ integer?) #(-> % count (= 1))))
  [:a 3])
=> {:tag :a, :children [3]}

https://clojure.github.io/spec.alpha/clojure.spec.alpha-api.html#clojure.spec.alpha/&

gonna have to test this before turning out, thanks

yes that's exactly it, that combinator flew under my radar, thanks @misha