rintro rfl
case mk.cons
α✝ : Type u_1
A' : Array α✝
a head✝ : α✝
tail✝ : List α✝
⊢ { data := List.dropLast (head✝ :: tail✝) } = A' →
    { data := head✝ :: tail✝ }[List.length tail✝] = a → size A' = List.length tail✝
case mk.cons
α✝ : Type u_1
a head✝ : α✝
tail✝ : List α✝
⊢ { data := head✝ :: tail✝ }[List.length tail✝] = a →
    size { data := List.dropLast (head✝ :: tail✝) } = List.length tail✝