Save a screen.
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 Syntax

         GT_saveScr([<nTop>], [<nLeft>], [<nBottom>], [<nRight>])
                                                               --> cScr

 Arguments:

       <nTop>    - Top line of screen area to save,
                   defaults to 0
       <nLeft>   - Leftmost col. of screen area to save,
                   default to 0
       <nRight>  - Rightmose col. of screen area to save,
                   defaults to maxrow()
       <nBottom> - Bottom line of screen area to save,
                   defaults to maxcol()

 Returns:

       cScr      - A string consisting og position information +
                   saved screen as a character string

 Description:

       Save screen (or partial screen) with position

       Would have been nicer to save as an array but that would
       have wasted too much memory.

       NOTE: If you use C, be aware that the return string
             can and often does contain embedded NULs

 Examples:

 Source: SAVESCR.PRG

 Author:  Andy M Leighton