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


DEBUG:Going to execute:
Ltac.cut X2
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = a
of type
constr of type tacvalue