Goal:
  
  A : Type
  forallb_InP : ∀ l : list A, (∀ x : A, In x l → bool) → bool
  x : A
  xs : list A
  H : ∀ x0 : A, In x0 (x :: xs) → bool
  ============================
   (x = x ∨ In x xs)


DEBUG:Going to execute:
match goal with
           | H:?X
             |- _ =>
                 match goal with
                 | H':?Y
                   |- _ =>
                       match H with
                       | H' => fail 2
                       | _ => unify X Y; clear H' || clear H
                       end
                 end
           end