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)
  hg : is_true (g' (f' n a))
  hf : is_true (forallb g (mapi_rec f l (S n)))
  H : ∀ x : A, nth_error (a :: l) 0 = Some x → g (f n x) → g' (f' n x)
  ============================
   (g' (f' n a) && forallb g' (mapi_rec f' l (S n)) = true)


DEBUG:Going to execute:
case not_locked_false_eq_true; Ltac.assumption
Level 1: Backtrace:

In environment
A : Type
B : Type
f, f' : nat → A → B
g, 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)
hg : g' (f' n a)
hf : forallb g (mapi_rec f l (S n))
H : ∀ x : A, nth_error (a :: l) 0 = Some x → g (f n x) → g' (f' n x)
Unable to unify "true" with
 "g' (f' n a) && forallb g' (mapi_rec f' l (S n))".
Level 0: Backtrace:

In environment
A : Type
B : Type
f, f' : nat → A → B
g, 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)
hg : g' (f' n a)
hf : forallb g (mapi_rec f l (S n))
H : ∀ x : A, nth_error (a :: l) 0 = Some x → g (f n x) → g' (f' n x)
Unable to unify "true" with
 "g' (f' n a) && forallb g' (mapi_rec f' l (S n))".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.