Goal:
    x : V.t  s : VSet.t  y : V.t  H0 : x = y -> False  H1 : x = y
  ============================
   (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 := H1
of type
intropattern
id0 := H0
of type
intropattern
flags := 
of type
tauto_flags
X2 := False
of type
constr of type tacvalue