Goal:
    P : elt -> elt -> Prop  s : t
  ============================
   ((forall x : elt, In x s -> forall x0 : elt, In x0 s -> P x x0) <->
    (forall v1 v2 : elt, In v1 s -> In v2 s -> P v1 v2))


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end