Muster: a reaction-diffusion painter. Two virtual chemicals react and spread across the screen following the Gray-Scott model. Every spot of the screen carries its own rule, a feed rate and a kill rate. Dragging paints the chosen rule onto the screen and seeds a little of the second chemical there, so coral, dividing cells, worms, mazes and waves grow side by side and into each other at their borders. Erase mode wipes the chemistry but keeps the rules. The toolbar picks the rule from a map, switches palette, speed and brush size, shows the painted rules, clears the picture, saves it as an image, copies a link and opens the help.