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


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := H0
of type
intropattern
id0 := hin
of type
intropattern
flags := 
of type
tauto_flags
X2 := False
of type
constr of type tacvalue