The different versions of the UN*X brand operating system are numbered
in a logical sequence: 5, 6, 7, 2, 2.9, 3, 4.0, III, 4.1, V, 4.2, V.2,
and 4.3.

From: http://www.cs.bgu.ac.il/~omri/Humor/unix-history.html

CS161