NAME
       CD_CHANGESTATE -- check if a "valid" disk is currently in a drive.

   FUNCTION
       This command checks to see if there is a "valid" disk in a drive.

   IO REQUEST INPUT
       io_Device       preset by the call to OpenDevice()
       io_Unit         preset by the call to OpenDevice()
       io_Command      CD_CHANGESTATE

   IO REQUEST RESULT
       io_Error - 0 for success, or an error code as defined in
                  <devices/cd.h>
       io_Actual - 0 means there is a disk while anything else indicates
                   there is no disk.

   NOTES
       A "valid" disk is a disk with a readable table of contents.