Software is different; it has no core. It is onionlike, a thing of layers, each built painstakingly and precariously on the previous one, each counting on the one below not to move or change too much. Software builders like to talk about laying bricks; skeptics see a house of cards. Either way, there's a steady accumulation going on. New layers pile on old. Programmers call these accretions "layers of abstraction," because each time a new one is added, something complex and specific is being translated into something simpler and more general... "This is what programmers do," wrote Eric Sink, a programmer who led the creation of the Web browser that became Microsoft's Internet Explorer. "We build piles of abstractions. We design our own abstractions and then pile them up on top of layers we got from somebody else." And every year the piles grow higher...