code: public XMLAttributeList addAttribute(@Nonnull String name, @Nonnull String value) {
          if (name == null) {
              throw new NullPointerException("name must be nonnull");
          }
          if (value == null) {
              throw new NullPointerException("value must be nonnull");
          }
          nameValuePairList.add(new NameValuePair(name, value));
          return this;
      }
docs: Add a single attribute name and value.
  
  @param name
  the attribute name
  @param value
  the attribute value
  @return this object (so calls to addAttribute() can be chained)