Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : EdgeSet.elt
  es : list EdgeSet.elt
  ih : VSet.In x (fold_right fold_fun VSet.empty es) <->
       (exists e : EdgeSet.elt,
          In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1))
  H : x = e..s \/ x = e..t
  H0 : VSet.In x (V G1)
  ============================
   ((e = e \/ In e es) /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1))


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 := x = e..s \/ x = e..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.