Interval basics

An interval [start, end] is every value from start to end, both ends included. Every interval question comes down to how two of them sit on the number line. Pick a chapter: one interval, two separate ones, the shapes of an overlap, merging two, their common part, and merging many after sorting by start. Each bar is a range, the axis under it is the number line; a green band is a stretch both share, a red one a gap that belongs to neither. The code is one reusable Range class.