Hmm... When you were talking I wasnt aware of discussion and made up some basis for this subclass. I send it with this mail.
Now Im gonna work on Intervals arithmetics to each other, for example: OpenedInterval from: 1 to: 5 right: true sumWith Interval from: 5 to: 8 should give Interval (or OpenedInterval) from: 1 to: 8 (both borders closed, so no difference).
After finishing this Im gonna try to make make some Interval converting, beginning from asOpenedInterval (what should give OpenedInterval with left and right false).