Goal:
    x : V.t  s : VSet.t  y : V.t  H0 : x = y  H : VSet.In y s  H1 : False
  ============================
   (x = y \/ VSet.In y s)


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