Goal:
  
  A : Type
  forallb_InP : ∀ (H := fixproto) (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:Tactics.fix_proto _ |- _ => Tactics.destruct_calls H; clear H
           end