Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x = y -> False
  a : V.t
  H0 : y = a
  H2 : VSet.In a p /\ y <> a
  ============================
   ((x = a \/ VSet.In a p) /\ y <> a)


DEBUG:Going to execute:
elim id
Level 3: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.In a p /\ y <> a
of type
constr of type tacvalue