feat(data/finset/basic): add card_insert_eq_ite (#10209)
feat(data/finset/basic): add card_insert_eq_ite (#10209)


Adds the lemma `card_insert_eq_ite` combining the functionality of `card_insert_of_not_mem` and `card_insert_of_mem`, analogous to how `card_erase_eq_ite`.





leanprover-community/mathlib,leanprover-community/mathlib