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)
  n : nat
  x : A
  H : nth_error l n = Some x
  ============================
   (g (f (0 + n) x) → g' (f' (0 + n) x))


DEBUG:Going to execute:
apply hfg
DEBUG LTAC Evaluated term: (∀ (n : nat) (x : A),
                              nth_error l n = Some x
                              → g (f (0 + n) x) → g' (f' (0 + n) x))