public function testSharing()
 {
     $this->backend->createAddressBook(self::UNIT_TEST_USER, 'Example', []);
     $books = $this->backend->getAddressBooksForUser(self::UNIT_TEST_USER);
     $this->assertEquals(1, count($books));
     $this->backend->updateShares('Example', [['href' => 'principal:principals/best-friend']], []);
     $shares = $this->backend->getShares('Example');
     $this->assertEquals(1, count($shares));
     // adding the same sharee again has no effect
     $this->backend->updateShares('Example', [['href' => 'principal:principals/best-friend']], []);
     $shares = $this->backend->getShares('Example');
     $this->assertEquals(1, count($shares));
     $books = $this->backend->getAddressBooksForUser('principals/best-friend');
     $this->assertEquals(1, count($books));
     $this->backend->updateShares('Example', [], [['href' => 'principal:principals/best-friend']]);
     $shares = $this->backend->getShares('Example');
     $this->assertEquals(0, count($shares));
     $books = $this->backend->getAddressBooksForUser('principals/best-friend');
     $this->assertEquals(0, count($books));
 }