Context is not defined as found on an example at Github

While trying to understand some the examples found on the "Beyond the Canvas" page on Github under the header "Element specific listeners", I continually get the error: "Uncaught ReferenceError: context is not defined." I have followed the instructions to a tee, (uncommented the necessary lines in index.html) and the puzzling thing is that when I comment out the line throwing the error, the code works as advertised. I also cannot find the function context in the other .js files, so I am wondering if context is left over from a previous version, or has yet to be added. Can anyone spread some light for me?

Thanks