Knowledge equals power... The string was important. After a while the Librarian stopped. He concentrated all his powers of librarianship. Power equals energy... People were stupid, sometimes. They thought the Library was a dangerous place because of all the magical books, which was true enough, but what made it really one of the most dangerous places there could ever be was the simple fact that it was a library. Energy equals matter... He swung into an avenue of shelving that was apparently a few feet long and walked along it briskly for half an hour. Matter equals mass. And mass distorts space. It distorts it into polyfractal L-space. So, while the Dewey system has its fine points, when you're setting out to look something up in the multidimensional folds of L-space what you really need is a ball of string.