@Override public int compare(SpanNode left, SpanNode right) {
      long x = left.span().timestampAsLong(), y = right.span().timestampAsLong();
      return (x < y) ? -1 : ((x == y) ? 0 : 1); // Long.compareTo is JRE 7+
    }
Convenience type representing a trace tree. Multiple Zipkin features require a trace tree. For
example, looking at network boundaries to correct clock skew and aggregating requests paths imply
visiting the tree.
zipkin/src/main/java/zipkin2/internal/SpanNode.java