Why here a arity errror and how to solve it

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6

It’s almost there:

(my-update-in {:a {:b 1}} [:a :b] inc)
;; => {:a {:b 2}}

(my-update-in {:a {:b 1}} [:a :b] + 3)
;; => throws arity exception

(update-in {:a {:b 1}} [:a :b] + 3)
;; => {:a {:b 4}}