docs: public @Nonnull Map<String, String> attr() {
          synchronized (this) {
              if (attrMap.get() == null) {
                  attrMap.set(
                      Optional.ofNullable(node.getAttributes())
                          .map(XQuery::attributesToMap)
                          .map(Collections::unmodifiableMap)
                          .orElseGet(Collections::emptyMap)
                  );
              }
          }
          return attrMap.get();
      }