Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  f' : nat → A → B
  g : B → bool
  g' : B → bool
  a : A
  l : list A
  IHl : ∀ n : nat,
          (∀ (n0 : nat) (x : A),
             nth_error l n0 = Some x → g (f (n + n0) x) = g' (f' (n + n0) x))
          → forallb g (mapi_rec f l n) = forallb g' (mapi_rec f' l n)
  n : nat
  hfg : ∀ (n0 : nat) (x : A),
          nth_error (a :: l) n0 = Some x
          → g (f (n + n0) x) = g' (f' (n + n0) x)
  n0 : nat
  x : A
  H : nth_error l n0 = Some x
  ============================
   (n + S n0 = S n + n0)


DEBUG:Going to execute:
lia