[Some tangential remarks on symbols
We'd certainly like to use the diamond symbol for possibility and the box symbol for necessity, but there is an issue here. We're going to choose one or more Unicode symbols ( 'Unicode... the most widely adopted software standard in the world...'), but web browsers use html so the question arises of Unicode to html conversion. As the development on web browsers continues, and html 4.0, more and more Unicode symbols are available, in fact all the 'mathematical' and 'logic' symbols are available on all modern browsers (see the wonderful Alan Wood) . However, of all things, the possibility diamond and the necessity square are not among the logic symbols. So we're in an awkward spot, we can choose a Unicode diamond and Unicode box, but in all probability someone somewhere on some computer with some browser will not be able to see both of them. We could use uppercase P for possibility and N for necessity (but that is a little drab). For the time being we'll go with the rather small and insipid lozenge '◊' ◊ which is Unicode 25CA and the also small Unicode white square 25A1 □ .These choices are not brilliant but they may be the most universal for just now.