Lemma about the coercion ℕ → WithBot ℕ.

An orphaned lemma about casting from ℕ to WithBot ℕ, exiled here to minimize imports to data.rat.order for porting purposes.

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theorem Nat.cast_withTop (n : ℕ) : ↑n = ↑n

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theorem Nat.cast_withBot (n : ℕ) : ↑n = ↑n