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Acknowledgements
 

The images in Chapters 6, 42, and 47 are by Stella Hui.

The definitions of Q , Q1 , and S in Chapter 7 were improved by a suggestion from Andrea Arbaci and Jay Foad.

The GIF at the beginning of Chapter 38 is by André Karwath in Wikimedia Commons.

The 3-door image at the beginning of Chapter 48 is by Cepheus in Wikimedia Commons.