Your moves are stored on the stack
The bottom value of a move is its direction
Any additional values are the indices of the boxes that were pushed
A red dash indicates the end of a move
A virtual undo takes the value at the bottom of the stack and moves the player in the opposite direction
Any additional values are interpreted as box indices and the corresponding boxes will be moved
The virtual undo continues until the stack is empty or a value with a red dash is read
A virtual undo reverses your most recent move using the values in the stack
A real undo reverts your most recent move or virtual undo
