intro hsame
x : ℤ
xs : List ℤ
⊢ List.length (x :: xs) = List.length [] → List.length (add (x :: xs) []) = List.length (x :: xs)
x : ℤ
xs : List ℤ
hsame : List.length (x :: xs) = List.length []
⊢ List.length (add (x :: xs) []) = List.length (x :: xs)