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 SCROLL()
 Scroll a screen region up or down, right or left
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 Syntax

     SCROLL([<nTop>], [<nLeft>],
        [<nBottom>], [<nRight>], [<nVert>] [<nHoriz>])
        --> NIL

 Arguments

     <nTop>, <nLeft>, <nBottom>, and <nRight> define the
     scroll region coordinates.  Row and columns values can range from 0, 0
     to MAXROW(), MAXCOL().  If you do not specify coordinate arguments, the
     dimensions of the visible display are used.

     <nVert> defines the number of rows to scroll, vertically.  A
     positive value scrolls up the specified number of rows.  A negative
     value scrolls down the specified number of rows.  A value of zero
     disables vertical scrolling.  If <nVert> is not specified, zero is
     assumed.

     <nHoriz> defines the number of rows to scroll horizontally.  A
     positive value scrolls left the specified number of columns.  A negative
     value scrolls right the specified number of columns.  A value of zero
     disables horizontal scrolling.  If <nHoriz> is not specified, zero is
     assumed.

     If you supply neither <nVert> or <nHoriz> parameters to SCROLL(), the
     area specified by the first four parameters will be blanked.

     Warning!  Horizontal scrolling is not supported on all of the
     alternate terminal drivers (i.e. ANSITERM, NOVTERM, PCBIOS).

 Returns

     SCROLL() always returns NIL.

 Description

     SCROLL() is a screen function that scrolls a screen region up or down a
     specified number of rows.  When a screen scrolls up, the first line of
     the region is erased, all other lines are moved up, and a blank line is
     displayed in the current standard color on the bottom line of the
     specified region.  If the region scrolls down, the operation is
     reversed.  If the screen region is scrolled more than one line, this
     process is repeated.

     SCROLL() is used primarily to display status information into a defined
     screen region.  Each time a new message is displayed, the screen region
     scrolls up one line and a new line displays at the bottom.

 Examples

     .  This user-defined function displays a message string at the
        bottom of a screen region after scrolling the region up one line:

        FUNCTION ScrollUp( nTop, nLeft, nBottom, nRight, ;
                             expDisplay )
           //
           SCROLL(nTop, nLeft, nBottom, nRight, 1)
           @ nBottom, nLeft SAY expDisplay
           //
           RETURN NIL

 Files:  Library is CLIPPER.LIB.