code: protected void attributeImpl(String name, String value) {
          if (! (this.xmlNode instanceof Element)) {
              throw new RuntimeException(
                  "Cannot add an attribute to non-Element underlying node: "
                  + this.xmlNode);
          }
          ((Element) xmlNode).setAttribute(name, value);
      }
docs: Add a named attribute value to the element for this builder node.
  
  @param name
  the attribute's name.
  @param value
  the attribute's value.