@Test
    public void combine_shouldReturnCombOfSpecifiedLength() {
        String word = "abcd";
        assertEquals("[[a], [b], [c], [d]]", combination.combine(word, 1).toString());
        assertEquals("[[a, b], [a, c], [a, d], [b, c], [b, d], [c, d]]", combination.combine(word, 2).toString());
        assertEquals("[[a, b, c], [a, b, d], [a, c, d], [b, c, d]]", combination.combine(word, 3).toString());
        assertEquals("[[a, b, c, d]]", combination.combine(word, 4).toString());

    }