A sender can tuck a few helper tiles beside the message. They travel together to the receiver.
One helper can remember a row’s promise: does it contain an odd or even number of lit squares?
Four lit squares: the helper promises EVEN.
Put one helper beside every row and below every column. Now each square has two checker directions.
Tap to place the helpers around the grid.
Try one flipped square. Its bad row and bad column cross in exactly one place.
cleanfliplocateheal
New test: two marked squares will flip together. They sit in different rows and different columns.
Everything else stays the same. What will the checker geometry reveal?
Two flips. Two row helpers. Two column helpers.
