@Test @Disabled void addNode_skipsOnCycle() {
    Span.newBuilder().traceId("a").parentId("d").id("b").name("s2").build();
    Span.newBuilder().traceId("a").parentId("b").id("d").name("s3").build();

    // TODO: see how spans like ^^ affect the node tree
  }
Same as {@link #constructsTraceTree()}, except with shared span ID
zipkin/src/test/java/zipkin2/internal/SpanNodeTest.java