The content of this wiki will be included in a forthcoming book from Chapman & Hall, and hence I need to own the copyright for the complete content of the wiki. The content of this site will continue to be freely available and I'll acknowledge all contributions in the printed book.
To confirm that you're ok with giving me your copyright, please include the phrase: "I assign the copyright of this contribution to Hadley Wickham".
If you have any questions, feel free to raise them in the pull request, or send me an email.