SL_VLINE()

  Short:
  ------
  SL_VLINE() Draws a vertical line from row,col to endrow,col

  Returns:
  --------
  cCode => control string to send to the printer

  Syntax:
  -------
  SL_VLINE(nTop,nLeft,nBottom,nRight,[cShade],[nDPIThick],[nLpi])

  Description:
  ------------
  Draws a vertical line from <nTop,nLeft> to <nBottom,nRight>. Based
  on rows and columns.

  [cShade] refers to the density of the line.

  The shading percentages are (default "100" - black) :

         "1 thru 2" = 2% shade
        "3 thru 10" = 10% shade
       "11 thru 20" = 20% shade
       "21 thru 35" = 30% shade
       "36 thru 55" = 45% shade
       "56 thru 80" = 70% shade
       "81 thru 99" = 90% shade
              "100" = 100% shade

  [nDPIThick] is the thickness in DotsPerInch of the
  line. Default is 2.

  [nLpi] is the lines per inch, and defaults to 6.

  Examples:
  ---------
   ??SL_VLINE(10,20,10,10,"50")  // draws a line 10,10 to 20,10 with
                                          // 50% shading

  Notes:
  -------
  Does not send anything to the printer - instead
  returns a control string that you send to the printer. String can be
  sent with ? or ?? or  QOUT() or QQOUT().

  For HP and compatible(PCL) Laserjet printers.

  Source:
  -------
  S_HPLAS.PRG