Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x = y -> False
  a : V.t
  H0 : y = a -> False
  H2 : x = a
  H1 : y = a
  ============================
   False


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