Procedure VertSlideBar(Details:SlideBarInfo)

    A window may have a slide bar.

    (Horizontal slide bars are similar except they lie horizontally.)

    This is usually used for text in a  window.  The user can either drag
    the button up and down (say to  scroll  the text), or click on the up
    arrow (to move up 1 line) or the down arrow (to move down 1 line).

    The user may also click anywhere on the bar to move the button there.
    There is a similar system in the Turbo Pascal 6 IDE.

    The slide bar on a window is used as follows:

    Details required are sent in the variable Details.

    The  coordinates  must  be   specified   in   X1,   Y1,  X2  and  Y2.
    A slide bar may only be 1 character in thickness.

    Forg and Back specify the slide bar colour.

    MainChar is the  character  used  to  draw  the  bar.   ButtonChar is
    the  character  used  to  display   the  button  on  the  slide  bar.
    UpLeftChar is used to  specify  the  character  used for the up arrow
    (on the  vertical  bar)  or  left  arrow  (on  the  horizontal  bar).
    DownRightChar is used to  specify  the character used for the down or
    right arrows on the bar.

    There is a current position  and  a  maximum  position.   The current
    position is  a  value  from  0  to  MaxPos.   That  is,  when  CurPos
    is 0, the button is at  the  top  (horizontal  left)  and  when it is
    MaxPos it is at the bottom (horizontal right).

    ButtonScreenPos,   ButtonNum,   BarButtonNum,   ButtonLeftUpNum   and
    ButtonRightDownNum  may  be  left  undefined  -  they are used by the
    object for screen positions and button numbers.