Goal:
    A : Type  B : Type  f : nat → A → B  l : list A  l' : list A
  ============================
   (mapi_rec f (l ++ l') 0 =
    mapi_rec f l 0 ++ mapi_rec (λ (n : nat) (x : A), f (#|l| + n) x) l' 0)


DEBUG:Going to execute:
rewrite mapi_rec_app