Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  n : nat
  x : A
  Hnth : nth_error (a :: l) n = Some x
  HPl : is_true (forallb p (a :: l))
  IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
  ============================
   (p x = true)


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

In environment
A : Type
p : A → bool
a : A
l : list A
n : nat
x : A
Hnth : nth_error (a :: l) n = Some x
HPl : forallb p (a :: l)
IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
Unable to unify "true" with "p x".
Level 0: Backtrace:

In environment
A : Type
p : A → bool
a : A
l : list A
n : nat
x : A
Hnth : nth_error (a :: l) n = Some x
HPl : forallb p (a :: l)
IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
Unable to unify "true" with "p x".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.