Goal:
  
  x : V.t
  s : VSet.t
  y : V.t
  H0 : x = y
  H : VSet.In y s
  H2 : x = y -> False
  ============================
   (x = y \/ VSet.In y 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 := H2
of type
intropattern
flags := 
of type
tauto_flags
X2 := False
of type
constr of type tacvalue