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light every pane.

lights out in a leaded window: each pane you press turns, and so do the four it touches

Pressing a pane turns it and the four panes it shares a lead with, lit to dark or dark to lit. The order never matters, and a pane pressed twice is as if it was never pressed, so every window has a shortest set of presses: par is that number, worked out exactly by the card, which treats each pane as a switch that is on or off and the window as a set of equations. On a 3x3 window that shortest set is the only one; on a 5x5 window every pattern that can be lit at all can be lit four different ways.

Concepts: logic, parity.

An interactive you play with one thumb, on Wundr.