public boolean markFile(String filename) {
        return !files.add(filename != null ? filename : NULL_FILENAME);
    }
@Test
    void testBug63433() {
        SampleResult firstResult = new SampleResult();
        Assertions.assertFalse(firstResult.markFile("result.csv"), "Expected false on first call of markFile");
        Assertions.assertTrue(firstResult.markFile("result.csv"), "Expected true on second call of markFile");

        SampleResult secondResult = new SampleResult();
        Assertions.assertFalse(secondResult.markFile(null), "Expected false on first call of markFile with null");
        Assertions.assertTrue(secondResult.markFile(null), "Expected true on second call of markFile with null");
    }