elim_casts
case h
α : Sort u_1
n m : Nat
a : α
as : LList α n
bs : LList α m
⊢ (let_fun this := ⋯ ▸ (HList.cons a as).append bs;
    this) ≅
    HList.cons a
      (let_fun this := ⋯ ▸ HList.append as bs;
      this)
case h
α : Sort u_1
n m : Nat
a : α
as : LList α n
bs : LList α m
⊢ (HList.cons a as).append bs ≅ HList.cons a (⋯ ▸ HList.append as bs)