Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x = y -> False
  a : V.t
  H0 : y = a
  H1 : VSet.In a p
  H3 : y = a -> False
  ============================
   ((x = a \/ VSet.In a p) /\ y <> a)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := H0
of type
intropattern
id0 := H3
of type
intropattern
flags := 
of type
tauto_flags
X2 := False
of type
constr of type tacvalue