English
For any Num n, the cast of pred n to ℕ equals Nat.pred n.
Русский
Для любого Num n, привод предшественника к ℕ равен Nat.pred n.
LaTeX
$$$\\forall n : \\text{Num}, (\\operatorname{pred} n : \\mathbb{N}) = \\operatorname{Nat.pred} n$$$
Lean4
@[simp, norm_cast]
theorem cast_mul [NonAssocSemiring α] (m n) : ((m * n : PosNum) : α) = m * n := by
rw [← cast_to_nat, mul_to_nat, Nat.cast_mul, cast_to_nat, cast_to_nat]