al shift in open documentation licenses moving 
from the GFDL to the CC-BY-SA (i.e. Wikipedia).
Using the CC-BY-SA license maintains the book's 
strong copyleft tradition while making it even more straightforward 
for new authors to reuse this material as they see fit.<br />
<br />
I feel that this book serves an example of why open 
materials are so important to the future of education,
and want to thank Allen B. Downey and Cambridge University
Press for their forward looking decision to make the book avai