All examples in this Problem are grids consisting of two objects.
Adjacent-numbered pages: BP1118 BP1119 BP1120 BP1121 BP1122  *  BP1124 BP1125 BP1126 BP1127 BP1128
EX9124 shows a 9 square by 9 square grid. Take each tile to be 3 squares by 3 squares; there is a 3 tile by 3 tile checkerboard pattern. (One of these tiles is itself a checkerboard pattern; the other is all black squares.)
hard, nice, precise, allsorted, hardsort, grid, miniworlds
Jago Collins