public <T> Span nextSpanWithParent(SamplerFunction<T> samplerFunction, T arg,
    @Nullable TraceContext parent) {
    return _toSpan(parent, nextContext(samplerFunction, arg, parent));
  }
@Test void nextSpanWithParent() {
    Span span = tracer.nextSpanWithParent(deferDecision(), false, context);

    assertThat(span.context().parentId()).isEqualTo(context.spanId());
  }