Feedback requested: A sample implementation of L-systems in Haskell

I recently published a draft of an article I wrote that explores L-systems, specifically D0L systems, and would very much enjoy any feedback (positive or negative) of what I have written. http://www.filipallberg.se/l-system.html Primarily, the content which gives me pause is the mathematics revolving morphic words as I have yet to fully grok them in any greater sense than the one that I have used in my implementation.