/UNDEFINE
       /UNDEFINE=(identifier[,...])
       /NOUNDEFINE (D)

    Performs the same function as the #undefine preprocessor
    directive: it cancels a macro definition.

    The /UNDEFINE qualifier is useful for undefining the predefined
    C++ preprocessor constants. For example, if you use a
    preprocessor constant to conditionally compile segments of code
    specific to C++ for OpenVMS systems, you can undefine constants
    to see how the portable sections of your program execute. For
    example:

     /UNDEFINE="deccxx"

    When both /DEFINE and /UNDEFINE are present on the CXX command
    line, /DEFINE is evaluated before /UNDEFINE.