English
For m ∈ ENat and n ∈ ℕ with n ≠ 0, toNat(m) = n iff m = cast(n).
Русский
Для m ∈ ENat и n ∈ ℕ, n ≠ 0: toNat(m) = n ⇔ m = cast(n).
LaTeX
$$$\\\\forall m \\\\in \\\\mathbb{N}_{\\\\infty}, \\\\forall n \\\\in \\\\mathbb{N}, n \\\\neq 0 \\\\Rightarrow \\\\operatorname{toNat}(m) = n \\\\iff m = n^{\\\\mathrm{cast}}$$$
Lean4
theorem toNat_eq_iff {m : ℕ∞} {n : ℕ} (hn : n ≠ 0) : toNat m = n ↔ m = n := by induction m <;> simp [hn.symm]