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)


DEBUG:Going to execute:
match goal with
           | |- _ -> _ => intro; t_tauto_intuit
           | |- _ => t_reduce; t_solver
           end