So by the... by... time 1990 rolled around I had almost 20 years of these transcripts of... of class discussions that had taken place in this Concrete Mathematics course and it was natural then - almost sort of mandatory - that this legacy be passed on to other generations, so... so... I... I wanted to write the book, Concrete Mathematics, describing how the course had evolved and... and what had been learned. One of the most conscientious teaching assistants was Oren Patashnik who had not only transcribed the class sessions but then he had... he had drafted, I think, almost all the chapters of the book as handouts to the students so that they could... so that they could use it in... in one of the years that I wasn't teaching. So I... so I... I could go from his... from his notes and... and rewrite them but using his insights into pedagogy as I... as I did this book. And... and Ron Graham who was the most successful teacher of the... of the course when I was gone... he... he'd come in two or three times and the classes... the... the people loved him so much when they took it that the... the class... the Concrete Maths class for his years would have annual reunions afterwards, you know.  They... and... and he also then started teaching it at Princeton as well. So I had then this Concrete Maths book with two... two co-authors, Oren Patashnik and Ron Graham. Usually when I write a book I... I finish about one page a day. I mean if the book is... is 365 pages long it takes me a year, if it's 5... if it's 500 pages long it takes me a year and a half. In this case I... I overdid it and I... and I got... I got the 600 page book done in little less than a year and it was... it was not a sustainable working style but... but I really wanted so much to get back to The Art of Computer Programming that Jill gave me a year where I didn't have to be quite as... as... what you'd call responsible a husband as... as... you know, I would be allowed to, you know... less of a human being for a year so that I could get this project done and then I could make up for it some other time. So anyway that was a crash project, which was done under some strain but... but I had the stimulation of the... the beautiful mathematical material that was... that was... writing up and have... and have Oren Patashnik's draft which was... which was, you know, in a refreshing style that... that we could make this book some sort of California book. One of the... one of the most fun things about the Concrete Math book is a... is a feature that I had... I copied from Stanford's, well... early '70s Stanford University had... had a really innovative thing for prospective students. They put out a book called Approaching Stanford that was written by former students and... and in the... in the main part of the pages of Approaching Stanford it would have this... what you would expect to find from a university... telling how, you know, how great the place is, the weather and all this... faculty being great and the students and everything. And then in the margins there's this graffiti where the students are saying what it really is, you know, like, you know, Stanford dorms are like a zoo without a keeper or something like this, you know, and so... but they'll... they'll... they have the true clues about what it's like written by students. And Stanford put this out and sent this out with its literature to prospective students. And so I thought, okay, let's do this with the math book too, let's have graffiti in the math book, let's let the students comment what they think about the subject and, you know, and... it turned out that this idea certainly has caught on because the book Concrete Math... Let me say first of all, I... if anybody finds an error in any of my books I... I pay them for it, you know, if they're... if they're the first to find the error I send them a check. And any errors in this graffiti were caught immediately, it was clear that all the readers were reading the graffiti, the... the errors in the rest of the... of the book, you know, maybe it would take three or four years before somebody finds it but... but if there's anything at all that they could comment on in the graffiti section that was... that was done. And also the book has been translated into about 10 other languages and... and in almost all the cases the translators also translated the... the graffiti, they thought this was a vital part of the book. So... so the book doesn't take itself too seriously but it's still... this turned out to be a way to contribute also to the content of the book because the... one of the comments could be... could... could say, you know, you'd better skim this... these next few pages because you don't want, you know, bother so much with it until you've been through it once before. And then there were other, some of... some of them were pretty corny jokes but we censored them a little bit and so it was... anyway, on balance that was... that was... So... so we have a California book about mathematics, a book that shows the informal style of Stanford classes as well as what I think is a personal manifesto of the way to do mathematics at the... the way I think that I love to... to do... work with the... with the kind of maths that I needed to study computer programs at any rate.