Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  x : A
  Hnth : nth_error (a :: l) 0 = Some x
  ============================
   (let H := CoreTactics.block in
    forallb p (a :: l)
    → (∀ n : nat, nth_error l n = Some x → forallb p l → p x)
      → let H0 := CoreTactics.block in p x)


DEBUG:Going to execute:
<coq-core.plugins.ltac::revert@0> $1
Level 3: evaluation returns
revert H'
where
H' := Hnth
of type
constr
H := Hnth
of type
constr of type tacvalue