Goal:
    s : VSet.t  s'' : VSet.t  x : V.t  H0 : VSet.In x s  H : False
  ============================
   (VSet.In x s'')


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