tw_ScrXtr( scr, tp,lt, bm, rt, tN, lN, leng, wid )
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     Extract a region from a SAVESCREEN() variable. This function and
     the next two are used in twSlide(), twMove() and twSize().

       Arguments:

     scr  - 'C' SAVESCREEN() to extract from.

     tp   - 'N' Top of SAVESCREEN().

     lt   - 'N' Left of SAVESCREEN().

     bm   - 'N' Bottom of SAVESCREEN().

     rt   - 'N' Right of SAVESCREEN().

     tN   - 'N' Relative Top row to begin extract.

     lN   - 'N' Relative Left column to begin extract.

     leng - 'N' Length of extracted region.

     wid  - 'N' Width of extracted region.

       Return:

     Clipper SAVESCREEN() variable.

       Example:

     RESTSCREEN( SBOTTOM, SLEFT, SBOTTOM, SRIGHT,;
                tw_ScrXtr( WHIDDEN, STOP, SLEFT, SBOTTOM, SRIGHT,;
                SBOTTOM, SLEFT, 1, SWIDTH + 1 ))