@Test
void isLetter() {

    assertThat(ASCII.isLetter('0')).isFalse();
    assertThat(ASCII.isLetter('9')).isFalse();
    assertThat(ASCII.isLetter('@')).isFalse();
    assertThat(ASCII.isLetter('a')).isTrue();
    assertThat(ASCII.isLetter('z')).isTrue();
    assertThat(ASCII.isLetter('A')).isTrue();
    assertThat(ASCII.isLetter('Z')).isTrue();
}