Goal:
  
  A : Type
  B : Type
  P : nat → A → Type
  l : list A
  f : nat → A → B
  g : nat → A → B
  n : nat
  ============================
   (Alli P n l
    → (∀ (n : nat) (x : A), P n x → f n x = g n x)
      → mapi_rec f l n = mapi_rec g l n)


DEBUG:Going to execute:
induction 1; simpl