@Test
public void testNormalizeStringPlus() {

    String s1 = "s1";
    String s2 = "s2";
    String s3 = "s3";
    List<String> sa = Arrays.asList(s1, s2, s3);
    Collection<Object> sl = Arrays.asList(s1, s2, s3, new Object()); // must be skipped
    assertNull(StringPlus.normalize(new Object()));
    assertEquals(Collections.singletonList(s1), StringPlus.normalize(s1));
    assertEquals(sa, StringPlus.normalize(sa));
    assertEquals(sa, StringPlus.normalize(sl));
}