Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  f' : nat → A → B
  g : B → bool
  g' : B → bool
  l : list A
  hfg : ∀ (n : nat) (x : A), nth_error l n = Some x → g (f n x) → g' (f' n x)
  H : ∀ (n : nat) (x : A),
        nth_error l n = Some x → g (f (0 + n) x) → g' (f' (0 + n) x)
  ============================
   (forallb g (mapi_rec f l 0) → forallb g' (mapi_rec f' l 0))


DEBUG:Going to execute:
<coq-core.plugins.ltac::clear@0> $1