Tiling with digits
We all remember those "modern" clocks with their green digits. They are vintage now. We will try though to tile an infinite grid with them – asking each digit to place itself not inside the cells of the grid but on the edges. Here is a 2 that understands what we want: "Tiling the grid with digits" means that every "elementary grid segment" must be covered by exactly one "elementary digit segment" (the five elementary segments of the digit 2 above cover as required five elementary segments of the grid). No overlaps are admitted (two digits 8 cannot be placed side by side as they would share at least one elementary grid segment). In order to tile the grid, all the digits can be rotated 90 or 180 degrees (clockwise or not). But we don't accept the digits 4 and 7 to be flipped (like in a mirror). A last remark: two digits 1 cannot form a plus sign – but they can form a T , if they want: NO YES And now the chal