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


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