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

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

  The /UNDEFINE qualifier is useful for undefining the predefined HP
  C preprocessor constants.  For example, if you use a preprocessor
  constant (such as vaxc, VAXC, VAX11C, or vms) to conditionally
  compile segments of code specific to HP C for OpenVMS Systems, you
  can undefine constants to see how the portable sections of your
  program execute.  For example:

  /UNDEFINE="vaxc"

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

  The default is /NOUNDEFINE.