{
{   The purpose  of  this  request  is to open a permanent file utility backup
{ file.  The permanent file utility file identifier  is  initialized  on  this
{ request and should be used on subsequent input/output requests on the backup
{ file.
{
{       PUP$OPEN_BACKUP_FILE (BACKUP_FILE_LFN, OPEN_POSITION, FILE_ID, STATUS)
{
{ BACKUP_FILE_LFN: (input) This parameter specifies the  name  of  the  backup
{       file.  This must be either a mass storage, null, or magnetic tape file.
{
{ OPEN_POSITION:  (input)  This parameter specifies the position that the file
{       is to be opened at.  Either amc$open_no_positioning,  amc$open_at_boi,
{       or   amc$open_at_eoi.   If  the  backup  file  is  a  tape  file  then
{       amc$open_no_positioning means at the  start  of  the  last  file,  and
{       amc$open_at_boi means at the start of the first volume.
{
{ FILE_ID:  (output)  This  parameter  returns the permanent file utility file
{       identifier that is used on  all  subsequent  requests  on  the  backup
{       file.
{
{ STATUS: (output) This parameter returns the request status.
{
{       CONDITIONS: pue$invalid_device_type
{
{       IDENTIFIER: puc$pf_utility_id
{
