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


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 = y
of type
constr of type tacvalue