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#ifndef                  Conditional Compilation by Lack of Macro Definition

 #ifndef  identifier


    The #ifndef directive, along with #else and #endif, controls
    compilation in a selective manner.  These directives are used to
    identify a group of source lines which are included or excluded from
    the compilation based on whether a given macro is defined.  If the
    macro is not defined the group of lines following the line with
    #ifndef is compiled.

      Notes:    This directive achieves the same result as:

                      #if !defined(identifier).

                #ifndef directives can be nested up to 32 levels deep.