simp only [set, mem_cons] at h ⊢
α : Type u_1
head✝ : α
bs : List α
x✝¹ x✝ : α
h : x✝ ∈ set (head✝ :: bs) 0 x✝¹
⊢ x✝ ∈ head✝ :: bs ∨ x✝ = x✝¹
α : Type u_1
head✝ : α
bs : List α
x✝¹ x✝ : α
h : x✝ = x✝¹ ∨ x✝ ∈ bs
⊢ (x✝ = head✝ ∨ x✝ ∈ bs) ∨ x✝ = x✝¹