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


DEBUG:Going to execute:
case H
Level 13: evaluation returns
case H; clear H
where
save := fun tac H => let x := fresh in
                     intro x; tac H; rename x into H
        where
        H := H
        of type
        constr
of type
tacvalue
H := H
of type
constr of type tacvalue