Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  f' : nat → A → B
  g : B → bool
  g' : B → bool
  l : list A
  ============================
   (∀ 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))


DEBUG:Going to execute:
induction l; cbn
DEBUG LTAC Evaluated term: 0