Goal:
  
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  d : DisjointAdd x s0 s'
  a : VSet.elt
  H0 : VSet.In a s'
  H1 : VSet.E.eq a x -> False
  H3 : x = a \/ VSet.In a s0 -> VSet.In a s'
  H2 : x = a
  ============================
   (VSet.In a s0)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
Level 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.E.eq a x
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.