@Test
    public void testParseLetters() {
        Set<String> set = GrammarParser.parseLetters("nonTerminals A B C d word");
        assertEquals(5, set.size());
        assertTrue(set.contains("A"));
        assertTrue(set.contains("B"));
        assertTrue(set.contains("C"));
        assertTrue(set.contains("d"));
        assertTrue(set.contains("word"));
        assertFalse(set.contains("a"));
        assertFalse(set.contains("b"));
        assertFalse(set.contains("c"));
        assertFalse(set.contains("D"));
    }