Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : Edge.t
  s : VSet.t
  _a_ : x = e..t
  _b_ : VSet.In x (V G1)
  ============================
   ((x = e..s \/ x = e..t) /\ VSet.In x (V G1))


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear id
Level 3: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := Edge.t
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.