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     An undefined macro is not the same as a macro defined to be null.
 
     Both kinds of macros expand to a null string when invoked.
     However, a macro defined to be null is still considered to be
     defined when used with preprocessing directives such as !IFDEF.
     A macro can be undefined by using the !UNDEF directive.
     See: ◄Preprocessing Directives►
 
     To define a macro to be null:
 
        ■ In a makefile or TOOLS.INI, specify zero or more spaces
          between the equal sign (=) and the end of the line, as in:
 
               LINKOPTIONS =
 
        ■ On the command line or in a command file, specify zero or
          more spaces enclosed in double quotation marks ("), or
          specify the entire null definition enclosed in double
          quotation marks, as in either of the following:
 
               LINKOPTIONS=""
               "LINKOPTIONS ="
 
     To undefine a macro, use !UNDEF, as in:
 
               !UNDEF LINKOPTIONS
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