SimpleRevisionServer implements RevisionServer { public Revision head() { return head.get(); } @Inject  SimpleRevisionServer(@Nullable ConflictResolver conflictResolver,
			@Nullable ForeignKeyResolver foreignKeyResolver); Revision head(); void merge(Revision base, Revision fork); Subscription registerListener(final Runnable listener);  }
@Test public void unsetHeadIsEmpty() { assertEquals(Revisions.Empty, server.head()); }