public Span newChild(TraceContext parent) {
    if (parent == null) throw new NullPointerException("parent == null");
    return _toSpan(parent, decorateContext(parent, parent.spanId()));
  }
@Test void newChild() {
    TraceContext parent = tracer.newTrace().context();

    assertThat(tracer.newChild(parent))
      .satisfies(c -> {
        assertThat(c.context().traceIdString()).isEqualTo(parent.traceIdString());
        assertThat(c.context().parentIdString()).isEqualTo(parent.spanIdString());
      })
      .isInstanceOf(RealSpan.class);
  }