© 2002 David J. Eck.
Modifications © 2011 Stephen Schaub.
This is a free textbook. There are no restrictions on using or redistributing or posting on the web a complete, unmodified copy of this material. There are some restrictions on modified copies. To be precise: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no invariant sections, front cover text, or back cover text.