Interesting. The author concludes at the end saying
Even worse, you can't detect this error in some validators because they think that those missing components may appear afterward.
may be xmlspy is one of it. Have you checked with topologi schematron validator or with others. i guess refering w3c spec would be the best choice.
By the way..i see you have edited your post and added space between some letter to disable smilies that appear because of xml tags,instead you tick the check box that appear below the post submit button "Disable smilies in this post". That will remove the cause.