Overview
 About the Screen Functions:
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 The Expand Library now provides a number of screen handling functions.
 The Clipper documentation suggests the application developer only to
 use the Clipper Terminal system for any Video I/O, and the Expand Library
 respects  those guidelines as all functions presented in this section make
 use of the Clipper SaveScreen() and RestScreen() functions, and of the
 Expand Library Internal though documented XPdelAttr(), XPreplAttr() and
 XPinsAttr() functions which perform primitive operations on the Strings
 returned by SaveScreen(). They are written in Assembly and thus ensure the
 fastest possible handling on the strings.

 This makes it possible to have efficient Screen handling functions
 and respect the Nantucket guidelines concerning Video I/O. By respecting
 those guidelines, one ensure's oneself that not many things need to be
 adapted when a new Clipper or Terminal system will come out. Hopefully,
 very few changes need to be made even in a graphical environment!

 Note that these functions will therefore not work when the IBM terminal
 system (Clipper default) is not used, as some drivers will not support
 saving and restoring screens.