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


DEBUG:Going to execute:
intro