Learn Haskell by hacking on crit-bit trees

I think this is a cool project, but surely needs more of a preamble? Like what are crit-bit trees? [1,2,3,4]

[1] http://benlynn.blogspot.com/2007/04/trees-hash-tables-and-tr...

[2] http://en.wikipedia.org/wiki/Radix_tree

[3] http://cr.yp.to/critbit.html

[4] http://www.imperialviolet.org/binary/critbit.pdf