Goal:
  
  G : t
  z : V.t
  x : V.t
  y : V.t
  z0 : V.t
  p : PathOf G y z0
  e : EdgeOf G z0 z
  i : V.t
  IHp : VSet.In i (nodes G p) \/ i = z0
  H : VSet.In i (nodes G (PathOf_add_end p e))
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   ((x = i \/ VSet.In i (nodes G p)) \/ i = z0)


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 := 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.