Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  v : VSet.elt
  s : VSet.t
  E : VSet.mem v (V G1) = false
  ============================
   (VSet.In x s <-> x = v /\ VSet.In x (V G1) \/ VSet.In x s)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro; clear id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := t
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.
Level 1: Backtrace:

Tactic failure.