def empty_positions(&block)
      positions = []
      each_position do |row, column|
        next if get_cell(row, column)
        yield(row, column) if block_given?
        positions << [row, column]
      end
      positions
    end
def empty_positions(&block)
      positions = []
      each_position do |row, column|
        next if get_cell(row, column)
        yield(row, column) if block_given?
        positions << [row, column]
      end
      positions
    end
Returns the corners of the empty cells