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


DEBUG:Going to execute:
let pos H :=
            match goal with
            | |- ~ _ => posneg H
            | |- _ -> False => posneg H
            | |- _ => pospos H
            end
           in
           match type of H with
           | ~ _ => neg H
           | _ -> False => neg H
           | _ => (elim H; fail) || pos H
           end