Create a new rule, called “Perimeter”:

Each of the cells on the edge of the grid (those in the A and H rows, in addition to the ones in the 1 and 8 columns) are known as Border Cells.

In between the current second and third steps of the atomic action “Tick”, add the following step:

For each Runner that is not the Grue, if all three cells in that runner’s Spoor are a Border Cell and that runner’s location is also a Border Cell, set that runner’s location to a random non Border cell which has a value of “-” and which the Grue does not have as its location. Then, reduce the Capacity of the Stack for each runner that was moved in this way by 1.

In the rule “The Grue”, remove the following text:

If the Grue’s Location is changed and the Cell in the Grue’s new Location contains a Battery, the Gridmaster should process the Collect action as if requested by the Grue as described in the Power section of the rules.