Hack chapter 4 into a million pieces, scatter on floor, attempt to reassemble.
* src/en/book/ch04-branching-and-merging.xml: slice and dice, add TODOs.
* src/en/book/ch02-basic-usage.xml: update some xref tags.
Authored by: sussman 2007-12-07
Parent: [r2898]
Child: [r2900]
