Goal:
  
  P : elt -> elt -> Prop
  s : t
  H : forall v1 v2 : elt, In v1 s -> In v2 s -> P v1 v2
  x : elt
  H0 : In x s
  x0 : elt
  ============================
   (In x0 s -> P x x0)


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