simp only [set, mem_cons] at h ⊢
α : Type u_1
b : α
bs : List α
n : Nat
x✝¹ x✝ : α
h : x✝ ∈ set (b :: bs) (Nat.succ n) x✝¹
⊢ x✝ ∈ b :: bs ∨ x✝ = x✝¹
α : Type u_1
b : α
bs : List α
n : Nat
x✝¹ x✝ : α
h : x✝ = b ∨ x✝ ∈ set bs n x✝¹
⊢ (x✝ = b ∨ x✝ ∈ bs) ∨ x✝ = x✝¹