Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  a : V.t  H : x2 = a
  ============================
   (x1 = a)


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 := x2 = 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.