Goal:
    A : Type  B : Type  f : nat → A → B  a : A  l : list A
  ============================
   (f 0 a :: mapi_rec (λ (n : nat) (x : A), f (S n) x) l 0 =
    f 0 a :: mapi_rec (λ x : nat, f (S x)) l 0)


DEBUG:Going to execute:
easy0 || (solve [ intuition eauto  3 with core terms ])