// If there is not already a live cell at location (r,c),
    // create a new live cell at location (r,c) in my grid.
    // --------------------------------------------------------------
    public void addLiveCell (int r, int c) {
    	if (theGrid[r][c] instanceof LiveCell) {
    		System.err.print("Warning: There is already a live cell");
    		System.err.println(" at location ("+ r +","+c+ ")");
    		return;
    	}
    	
    	theGrid[r][c] = new LiveCell();
    	
    	CellInfoNode newCell = new CellInfoNode(r, c, true);
    	theInfoGrid[r][c] = newCell;
    	
    	// Insert the new CellInfoNode in the linked list.
    	newCell.next = theInfoList;
    	theInfoList = newCell;
    	
    	// Set the new CellInfoNode's neighbours to -1 to
    	// help simplify the counting of neighbours
    	newCell.numNeighbours = -1;
    }
    
    
    // Compute the next generation of cells.
    // --------------------------------------------------------------
    public void nextGeneration() {
    	CellInfoNode cCell = theInfoList;
    	while (cCell != null) {
    		// For the previous row to the following row...
    		for (int i = -1; i <= 1; i++) {
    			// Figure out which row and column we're dealing with.
    			// We must modulo with numRows() because we want to wrap
    			// around the board.
    			int r = (cCell.row + i + numRows()) % numRows();
    			
    			// For the previous column to the following column...
    			for (int j = -1; j <= 1; j++) {
    				int c = (cCell.col + j + numCols()) % numCols();
    				// If theInfoGrid is a CellInfoNode, add a neighbour.
    				if (theInfoGrid[r][c] instanceof CellInfoNode)
    					theInfoGrid[r][c].addNeighbour();
    				// else create an emtpy CellInfoNode and at it
    				// to theInfoList
    				else {
    					theInfoGrid[r][c] = new CellInfoNode(r,c,false);
    					theInfoGrid[r][c].next = theInfoList;
    					theInfoList = theInfoGrid[r][c];
    				}
    			}
    		}
    		cCell = cCell.next;
    	}
    	
    	// For each CellInfoNode, tell the Cell at that location in theGrid to evolve.
    	CellInfoNode pCell = null;
    	cCell = theInfoList;
    	while (cCell != null) {
    		theGrid[cCell.row][cCell.col] = theGrid[cCell.row][cCell.col].evolve(cCell.numNeighbours);
    		
    		// If after evolution it is alive, reset number of neighbours and move to
    		// next CellInfoNode in the list
    		if (theGrid[cCell.row][cCell.col] instanceof LiveCell) {
    			cCell.numNeighbours = -1;
    			pCell = cCell;
    		}
    		// After evolution the cell is dead, remove it from theGrid and theInfoList
    		// and move on to next CellInfoNode in the list
    		else {
    			theInfoGrid[cCell.row][cCell.col] = null;
    			if (pCell != null)
    				pCell.next = cCell.next;
    			else
    				theInfoList = cCell.next;
    		}
    		cCell = cCell.next;
    	}
    }