Goal:
  
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  x3 : V.t
  a : V.t
  H0 : x0 = a
  ============================
   (x0 = a \/ x1 = a \/ VSet.In a _z_)


DEBUG:Going to execute:
Ltac.assumption
Level 6: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x0 = a
of type
constr of type tacvalue

Level 6: Backtrace:

Tactic failure.
Level 6: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.