@Test
public void testSeal() {

    // PREPARE
    final String name = "Linux";
    final DownloadOSFamily testee = new DownloadOSFamily(name);
    final DownloadOS v16 = new DownloadOS("Ubuntu 16.04 64-bit (.deb)",
            "https://eventstore.org/downloads/ubuntu/xenial/eventstore-oss_5.0.0-rc1-1_amd64.deb");
    final DownloadOS v18 = new DownloadOS("Ubuntu 18.04 64-bit (.deb)",
            "https://eventstore.org/downloads/ubuntu/bionic/eventstore-oss_5.0.0-rc1-1_amd64.deb");
    testee.addOS(v16);
    // TEST & VERIFY
    testee.seal();
    try {
        testee.addOS(v18);
        fail();
    } catch (final IllegalStateException ex) {
        assertThat(ex.getMessage()).isEqualTo("The instance is sealed");
    }
}