INT 21 - Concurrent DOS v3.2+ - INSTALLATION CHECK
        AX = 4451h
Return: CF set if not Concurrent DOS
            AX = error code (see AH=59h)
        CF clear if successful
            AH = single-user/multiuser nature
                10h single-user
                    AL = operating system version ID (see AX=4452h)
                14h multiuser
                    AL = operating system version ID (see below)
Notes:  as of Concurrent DOS/XM 5.0 (possibly earlier), the version is stored
          in the environment variable VER
        use this function if you are looking for multiuser capabilities,
          AX=4452h for single-user
        this function should never return the single-user values
SeeAlso: AX=4452h,AX=4459h

Values for operating system version ID:
 32h    Concurrent PC DOS 3.2
 41h    Concurrent DOS 4.1
 50h    Concurrent DOS/XM 5.0 or Concurrent DOS/386 1.1
 60h    Concurrent DOS/XM 6.0 or Concurrent DOS/386 2.0
 62h    Concurrent DOS/XM 6.2 or Concurrent DOS/386 3.0
 66h    DR Multiuser DOS 5.1
 67h    Concurrent DOS 5.1

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