Writing is abundantly difficult without artificial constraints. Adding arbitrary limits to what you may scrawl -- such as to abandon that most common, fifth, symbol of our traditional Latin orthography, or lay down only so many words within pairs of stops -- without doing so ghastly an injury to grammar or signification as to crowd out all worth is in my opinion too hard to do. You may not concur; if not, visit this for compositions surpassing this slight try. You can add your own, too.
(OK, so that was a plug. I'm no Oulipo fan, particularly. But thanks to Mark for drawing it to my sight.)