Drawing to the screen – albeit repeatedly at superhuman speeds – is the root of screen-based, computer generated, form. Couching computation in terms of the manipulation of things that leave marks is an approach to making programming more approachable, and is similar, if not the same as, the drawing done in LOGO via the command controlled movement of the turtle. For the purposes of the model presented in this thesis it may provide a way to partially accommodate the plasticity of the screen space.