z -- do special extended things

        default range               syntax
    lines must be given         line,line z <option> <stuff>

       The z command command is used for doing special extended
    things. Currently, the only option available is b, to  draw
    a box into the buffer. It works as follows:

         lin,lin zb<left>[,<right>][<char>]

    The two <lin>s  specify where in the buffer to put the  box,
    <left> and <right> tell which column to  which  column,  and
    the <char> is the character to use to draw the  box.  As  an
    example,
         .,.+4zb10,50*
    draws a box, OVER the text that is there. Try  it  on  some
    BLANK lines.