-The "world" is a 7x9 grid with three "birds" moving in a straight line
-and bouncing on the world's borders. The colors correspond to a fixed
-"z-buffer order". It could be another "world", but this one has
-objectness, occlusion, and motion.
+The "world" is a 6x8 grid with three "birds" moving in a straight line
+and bouncing on the world's borders. It could be another "world", but
+this one has objectness and motion. There are ten colors and 4
+directions of motions, so roughly (6x8x4x10)**3 ~ 7e9 states.