def []=(x, y, content)
      if content.nil?
        @store.delete([x, y]) && nil
      else
        @store[[x, y]] = content
      end
    end
def []=(x, y, content)
      if content.nil?
        @store.delete([x, y]) && nil
      else
        @store[[x, y]] = content
      end
    end
Putting new content in a cell.