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 : ∀ x : A,
          nth_error (a :: l) 0 = Some x → g (f (n + 0) x) = g' (f' (n + 0) x)
  ============================
   (g (f n a) = g' (f' n a))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: (hfg 0)