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


DEBUG:Going to execute:
repeat
            match goal with
            | H:context [ ?P -> False ] |- _ => fold (~ P) in H
            | |- context [ ?P -> False ] => fold (~ P)
            end