def update_row idx, *cells
      res = if row = @rows[idx]
              row[0, cells.size] = cells
              row
            else
              Row.new self, idx, cells
            end
      row_updated idx, res
      res
    end
def update_row idx, *cells
      res = if row = @rows[idx]
              row[0, cells.size] = cells
              row
            else
              Row.new self, idx, cells
            end
      row_updated idx, res
      res
    end
Updates the Row at _idx_ with the following arguments.