Some fanciful properties for clock

Yes in these cases we need custom generators. And it will also solve the `?IMPLIES` duplication because you can define your custom domain without the need of taking a large domain and then restrict it with the `?IMPLIES`. [Here](http://erlcode.wordpress.com/2010/12/05/automated-testing-%E2%80%93-bringing-out-the-big-guns-%E2%80%93-part-3/) explains how to do it.