{
{   The purpose of this request is to unpack a record header located in}
{ the user's buffer_area. The buffer_area itself may be located within}
{ a file opened with amc$segment access_level or in a space allocated}
{ by the program.}
{   This request may be used to unpack record headers built by amp$put}
{ requests or amp$pack_record_header.}
{   This request may only be issued for a file opened with amc$segment or}
{ amc$physical access_level.}
{   If this request is issued for a file which contains records which are}
{ not preceded by a record header, normal status is returned and the}
{ buffer_area variable is returned unchanged.}
{   This request performs a NEXT in the buffer_area sequence. When this}
{ request completes, the buffer_area variable contains a pointer to the}
{ first byte beyond the record header in the buffer. If insufficient}
{ space existed in the buffer for the record header, a NIL pointer is}
{ returned in the buffer_area variable along with abnormal status.}
{
{       AMP$UNPACK_RECORD_HEADER (FILE_IDENTIFIER, BUFFER_AREA, HEADER, STATUS)
{
{ FILE_IDENTIFIER: (input) This parameter specifies the file access
{       identifier established when the file was opened.
{
{ BUFFER_AREA: (input-output) This parameter specifies the location within}
{       the user buffer of the record header which is to be unpacked. This}
{       request returns the address of the first byte beyond the record}
{       header in the buffer. A NIL pointer and abnormal status are returned}
{       if a full record header was not available in the buffer. This request}
{       assumes that the record header and the record have been stored}
{       contiguously in the buffer prior to issuing this request.}
{}
{ HEADER: (output) This parameter specifies the contents of the record}
{       header which has been unpacked form the buffer_area. This parameter}
{       must be supplied but will be ignored if the record_type of the}
{       file is one which does not support a record header.}
{       Not all fields of this parameter are applicable to record types other}
{       than amc$variable; no indication is given if such fields are not}
{       initialized by this request.}
{       This parameter is a record which includes the following fields:}
{        Header_type - specifies the type of record header which has been}
{                      unpacked.}
{        Length - specifies the number of bytes in the record described by}
{                 this header.}
{        Previous_length - specifies the number of bytes in the preceding}
{                          record, exclusive of the previous header.}
{        Unused_bit_count - specifies the number of bits in the last byte of}
{                           this record which do not contain valid data.}
{        User_information - specifies a cell whose value is defined only if}
{                           the record header was created by amp$pack_record_}
{                           header.}
{}
{ STATUS: (output) this parameter specifies the request status.
{       CONDITION:  ame$ring_validation_error,
{       IDENTIFIER: amc$access_method_id.
{
