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


DEBUG:Going to execute:
fun tauto_flags X1 => <coq-core.plugins.tauto::is_disj@0>