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
           | H:?x = ?x |- _ => clear H
           | H:?x = ?y |- _ => rewrite -> H in *; clear H
           end