Goal:
    G : t  x : V.t  i : V.t  source_vertex0 : VSet.In (s G) (V G)  H0 : x = i
  ============================
   (i = x)


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