Critters is a visual programming language that I worked on in grad school. These documents were written in that context rather than for a general audience.
The first implementation was written in the summer of 1995 using STk, an object-oriented dialect of Scheme with bindings for the Tk toolkit. This served as a proof of concept for the thesis proposal.
The second implementation was mostly written in summer 1998. It was written in C with a custom dynamic object system, using OpenGL for rendering and Tk for the user interface. It included many of the features that were originally described in the proposal. It could simulate a bouncing ball with a fixed timestep, using programmable rules for motion, bouncing, and gravity.