Goal:
  
  G : t
  x : V.t
  i : VSet.elt
  H : VSet.In i (VSet.add x VSet.empty)
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In i VSet.empty \/ i = x)


DEBUG:Going to execute:
match goal with
           | H:?T
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           | H:forall x : ?T, _
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           end