the line pointer for the line to be positioned at the top of screen when the macro exits.
Returns:
Both num-var and RC will be 0 and Msg$ will be "" (Null) for successful completion
Both num-var and RC will be 8 and Msg$ set to an error message on failure (e.g. invalid line pointer)
Special Notes:
If your macro uses both Set_TopScrn_LPtr and Set_Csr, the positioning of Set_Csr will override that of the Set_TopScrn_LPtr if the desired cursor position is not within the visible screen as set by the Set_TopScrn_LPtr function. i.e. Set_Csr 'wins'.