public boolean debug() {
    return (flags & FLAG_DEBUG) == FLAG_DEBUG;
  }
@Test void debug() {
    MutableSpan span = new MutableSpan();
    span.setDebug();
    assertThat(span.debug()).isTrue();

    span.unsetDebug();
    assertThat(span.debug()).isFalse();
    assertThat(span.flags).isZero();
  }