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


DEBUG:Going to execute:
Ltac.assumption
Level 7: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x1 = a
of type
constr of type tacvalue

Level 7: Backtrace:

Tactic failure.
Level 7: Backtrace:

Tactic failure.
Level 6: Backtrace:

Tactic failure.
Level 6: Backtrace:

Tactic failure.