Goal:
    s : VSet.t  x : VSet.elt  H : VSet.In x s  y : VSet.elt
  ============================
   (VSet.In y s -> x = y \/ VSet.In y s)


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