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:
intro x; tac H
Level 14: evaluation returns
intro x; tac H; rename x into H
where
x := H0
of type
intropattern
tac := fun H => 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
H := H
of type
constr of type tacvalue

Level 13: evaluation returns
intro x; tac H; rename x into H
where
x := H0
of type
intropattern
tac := fun H => 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
H := H
of type
constr of type tacvalue