Then I come back home and I'm thinking, okay, I'm ready… I'm going to do Volume Four, and I… I… Volume Four is about combinatorial algorithms. Now, combinatorial means… deals with zillions of combinations of ways to do things, and… and as a result there's… there were many, many problems where people… people had been wanting computers to solve, because they couldn't solve them. So many cases had to be done. You needed a computer to do it, yet nobody could figure out how to do it efficiently. And so, a… a good idea could… could make a… a method run more than a million times faster, and I was collecting all these good ideas but people kept having more and more good ideas. And so, there's something in the field called combinatorial explosion, which means to most people that the size of a problem is growing… is growing huge, very rapidly. To me, combinatorial explosion meant the research on combinatorial methods was growing explosively. In 1974, five and six, when I'm working on Volume Four, more than 50% of all the papers in all the technical journals were about things that belonged in Volume Four. So, in other words, it's like sitting on top of a kettle that's boiling, you… you can't control it. Every time… if I'd write something one week, it would be obsolete the next week. It's like, you know, trying to write a book about the internet today, or something, you know. So, it looked impossible to finish Volume Four. I… doing my best and, you know… gathering material for it and… and reading section, making lots of notes, and upstairs I have hundreds of folders of… of these… of these notes. I started… I started and I had 30 folders and they were well-organized, and then I made folders called X1, X2, X3 until I got up to X15 – not well-organized but just extensions to the system – and then as new material came in I started throwing it into a pile, saying I hope to have time to read this some day. So I was… just… field is growing very fast.