It would be helpful to be able to get added items out again without having to result to casting and depending on impl-base.

e.g:
EnterpriseArchive ear = ShrinkWrap.create(EnterpriseArchive.class)
.addModule(ShrinkWrap.create(WebArchive.class, "my.war"));

WebArchive war = ear.get(WebArchive.class, ArchivePaths.create("/my.war"))