Copyright (C) Allen B. Downey, Paul Bui, Jeffrey Elkner,
and Dafni Hernandez. Permission is granted to copy, distribute and/or
modify this document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software Foundation;
with Invariant Sections being Preface and Contributor List, no Front-Cover
Texts, and no Back-Cover Texts. A copy of the license is included in the
section entitled “GNU Free Documentation License”.