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.