code: def border_width=(width)
           size = width.nil? ? 0 : width
           if size > 0
              @borders[TOP] = @borders[RIGHT] = @borders[BOTTOM] = @borders[LEFT] = size.to_i
           else
              @borders = [nil, nil, nil, nil]
           end
        end
docs: This method assigns a width, in twips, for the borders on all sides of
   the cell. Negative widths will be ignored and a width of zero will
   switch the border off.
  
   ==== Parameters
   width::  The setting for the width of the border.