docs: public Whitelist addAttributes(String tag, String... attributes) {
          Validate.notEmpty(tag);
          Validate.notNull(attributes);
          Validate.isTrue(attributes.length > 0, "No attribute names supplied.");
  
          TagName tagName = TagName.valueOf(tag);
          tagNames.add(tagName);
          Set<AttributeKey> attributeSet = new HashSet<>();
          for (String key : attributes) {
              Validate.notEmpty(key);
              attributeSet.add(AttributeKey.valueOf(key));
          }
          if (this.attributes.containsKey(tagName)) {
              Set<AttributeKey> currentSet = this.attributes.get(tagName);
              currentSet.addAll(attributeSet);
          } else {
              this.attributes.put(tagName, attributeSet);
          }
          return this;
      }