Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : VSet.In x s -> False
  H : VSet.In x s \/ VSet.In x s''
  ============================
   (VSet.In x s'')


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