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


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; Ltac.split
Level 2: evaluation returns
<coq-core.plugins.tauto::find_cut@0>
where
k := fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]
     where
     flags := 
     of type
     tauto_flags
of type
tacvalue of type tacvalue

Level 2: Backtrace:

Tactic failure: No matching clause.
Level 2: Backtrace:

Tactic failure: No matching clause.
Level 2: Backtrace:

Tactic failure: No matching clause.
Level 1: Backtrace:

Tactic failure: No matching clause.