⊢ list.nil = list.nil

h : ℕ,
t : list ℕ,
iH : list.append t list.nil = t
⊢ list.append (h :: t) list.nil = h :: t
------
h : ℕ,
t : list ℕ,
iH : list.append t list.nil = t
⊢ h :: list.append t list.nil = h :: t