Goal:
    A : Type  H : ∀ x : A, In x [] → bool
  ============================
   (∀ (f : A → bool) (l : list A) (g : ∀ x : A, In x l → bool),
      g = (λ (x : A) (_ : In x l), f x)
      → {| pr1 := []; pr2 := H |} = {| pr1 := l; pr2 := g |}
        → true = forallb_InP l g
          → let H := CoreTactics.block in forallb_InP l g = forallb f l)


DEBUG:Going to execute:
let H := fresh x in
           intro
           H
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.