Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  v : VSet.elt
  s : VSet.t
  E : VSet.mem v (V G1) = false
  H : VSet.In x s
  ============================
   (VSet.In x (V G1))


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