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:
solve [ Ltac.left; t_tauto_intuit | Ltac.right; t_tauto_intuit ]
Level 2: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = e..s /\ VSet.In x (V G1) \/
      x = e..t /\ VSet.In x (V G1) \/ VSet.In x s
of type
constr of type tacvalue