@Test
void isDigit() {

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