BACKSPACE

 Positions a sequential file at the beginning of the preceding
 record, making it available for subsequent I/O processing.  The
 file must be on disk or tape.  Statement format:

     BACKSPACE ([UNIT=]io-unit [,ERR=label] [,IOSTAT=i-var])
     BACKSPACE io-unit

     io-unit  Is an integer variable or constant specifying the 
              logical unit number of the file, optionally prefaced 
              by UNIT=.  UNIT= is required if unit is not the 
              first I/O specifier.

     label    Is the label of a statement that receives control 
              if an error occurs, prefaced by ERR=.

     i-var    Is a scalar integer variable to which the completion 
              status of the I/O operation is returned, prefaced 
              by IOSTAT= (positive if an error occurs, zero if 
              no error occurs).

 A BACKSPACE statement should not be specified for a file that is
 open for direct, append, or keyed access, because record "n" is not
 available to the RMS I/O system.

 If a file is already positioned at the beginning of a file, a
 BACKSPACE statement has no effect.