def count_filled_positions(self):
        """
        return a count of blank cells
        """
        filled = 0
        for row_ndx in range(self.grid_height - 0):
            for col_ndx in range(self.grid_width - 0):
                if self.get_tile(row_ndx, col_ndx) != EMPTY:
                    filled += 1
        return filled
return a count of blank cells