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#ifdef                   Conditional Compilation by Macro Definition

 #ifdef  identifier

    The #ifdef 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 defined the group of lines following the line with #ifdef is
    compiled.

      Notes:    This directive achieves the same result as:

                      #if defined(identifier)

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