feat(set_theory/cardinal/ordinal): some lemmas about adding finite cardinals and (in)equalities (#16262)
feat(set_theory/cardinal/ordinal): some lemmas about adding finite cardinals and (in)equalities (#16262)

These lemmas arose from [this Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there-code-for-X.3F/topic/cardinal.2Ele_of_add_le_add).

Comments are welcome!





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