clojure.spec - reducing repetition with &

Remove the repetition here without nesting or additional keywords in the conformed data?

(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))(s/def ::fact-without-var
  (s/cat :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))

doesn't the first one include the second?
But I want a spec for data where a var is forbidden. And one where it's optional.
ah
so, you can create a new spec that is an s/cat that just contains the common portions, then reference it in both ::fact specs

(s/def ::fact-without-var
  (s/cat :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :tail ::fact-without-var))

that does introduce nesting in the conformed data so is not everything you want
another way would be to restrict the bigger one
maybe something like

(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))(s/def ::fact-without-var
  (s/& ::fact #(nil? (:var %))))

can't say I've done this before, might need to tweak that predicate more

& just lets you and arbitrary predicates into the regex spec

Review the ::fact and ::fact-without-var specs in the issue, focusing on the behavior of s/cat, s/def, and s/&. Determine whether the repeated portions can be shared while preserving flat conformed data and separate handling of optional versus forbidden vars; done means the intended spec behavior is established.