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
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end