Goal:
  
  x : V.t
  p : VSet.t
  H : VSet.In x p -> False
  a : V.t
  H3 : x = a -> False
  H1 : x = a
  H2 : False
  ============================
   (VSet.In a p)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := H1
of type
intropattern
id0 := H3
of type
intropattern
flags := 
of type
tauto_flags
X2 := False
of type
constr of type tacvalue