The numeric branch for this version. This reflects the fact that most software versions are tree-structured; there is a main trunk which is tagged with versions at various points (1,2,3...), and the first branch off the trunk after version 3 is 3.1, the second branch off the trunk after version 3 is 3.2, and so on. The tree can be branched arbitrarily, just by adding more digits.
We represent the branch as a list of Int, so version 3.2.1 becomes [3,2,1]. Lexicographic ordering (i.e. the default instance of Ord for [Int]) gives the natural ordering of branches.