{
{   The purpose  of  this  procedure  is  to both attach and open a volume set
{ table for a particular volume.  The stored version name is compared with the
{ current vst version name.
{
{       STP$OPEN_VST (VSN, DATA_TO_BE_MODIFIED, SFID, VST_SEGMENT_POINTER,
{         STATUS)
{
{ VSN: (input) This parameter specifies the volume whose volume set  table  is
{       to be opened.
{
{ DATA_TO_BE_MODIFIED: (input) This parameter specifies whether the requestor
{      is going to modify the volume set table or merely read the volume
{      set table.
{
{ SFID: (output)  This  parameter  returns  the system file identifier for the
{       volume set table device file.
{
{ VST_SEGMENT_POINTER: (output) This parameter returns the segment pointer  to
{       the  volume  set  table.   This  may be used to directly reference the
{       volume set table.
{
{ STATUS: (output) This parameter returns the request status.
{
{       CONDITIONS: ste$attach_df_error
{                   ste$vol_not_in_set
{                   ste$open_error
{                   ste$vol_set_table_lost_data
{                   ste$incompatible_vst_version
{
{       IDENTIFIER: stc$set_management_id
{
