There aren't any silver bullets in software development, and there probably never will be. However, Extreme Programming is a simple set of common-sense practices that, when used together, really can give you much of what you just read in the paragraph above. In this book, we tell you what the XP practices are, and how to install them in your project.

Numerical results are presented indicating dc = 4 as the upper critical dimension for the Bak-Sneppen evolution model. This finding agrees with previous theoretical arguments, but contradicts a recent Letter [Phys. Rev. Lett. 80, 5746-5749 (1998)] that placed dc as high as d = 8. In particular, we find that avalanches are compact for all dimensions d ≤ 4, and are fractal for d> 4. Under those conditions, scaling arguments predict a dc = 4, where hyperscaling relations hold for d ≤ 4. Other properties of avalanches, studied for 1 ≤ d ≤ 6, corroborate this result. To this end, an improved numerical algorithm is presented that is based on the equivalent branching process. PACS number(s): 64.60.Ak, 05.40.-a, 05.65.+b. The Bak-Sneppen evolution model [1] has received considerable attention as an archetype of self-organized criticality [2], which has been put forward as a general mechanism leading to many non-equilibrium scaling phenomena observed in Nature [3]. Although the Bak-Sneppen model was originally developed as an attempt to interpret paleontological data indicating co-evolutionary activity