use N, |a| + 1
case intro
s : ℕ → ℝ
a : ℝ
cs : ConvergesTo s a
N : ℕ
h : ∀ n ≥ N, |s n - a| < 1
⊢ ∃ N b, ∀ (n : ℕ), N ≤ n → |s n| < b
case h
s : ℕ → ℝ
a : ℝ
cs : ConvergesTo s a
N : ℕ
h : ∀ n ≥ N, |s n - a| < 1
⊢ ∀ (n : ℕ), N ≤ n → |s n| < |a| + 1