@Override
    public int hashCode() {
      int h$ = 1;
      h$ *= 1000003;
      h$ ^= left.hashCode();
      h$ *= 1000003;
      h$ ^= right.hashCode();
      return h$;
    }
links are merged by mapping to parent/child and summing corresponding links
zipkin/src/main/java/zipkin2/internal/DependencyLinker.java