Goal:
  
  A : Type
  B : Type
  f : nat → A → bool
  g : nat → B → A
  n : nat
  l : list B
  ============================
   (alli f n (mapi_rec g l n) = alli (λ (i : nat) (x : B), f i (g i x)) n l)


DEBUG:Going to execute:
revert n; induction l => n; simpl