From the site:
'Chris Coyne created a small language for design grammars. These grammars are sets of non-deterministic rules to produce images. The images are surprisingly beautiful, often from very simple grammars.
Chris' program was a command line program for unix-like systems. We couldn't wait to play with creating our own images, but we wanted a full graphical environment for exploring them. Hence, Context Free, an environment for editing and rendering CFDG design grammars.'