public Span newTrace() {
    return _toSpan(null, newRootContext(0));
  }
@Test void newTrace_resultantSpanIsLocalRoot() {
    Span span = tracer.newTrace();

    assertThat(span.context().spanId()).isEqualTo(span.context().localRootId()); // Sanity check
    assertThat(span.context().isLocalRoot()).isTrue();
  }