Section 13,9: BACKSPACE

 Repositions a sequential file that is currently open for sequential
 access to the beginning of the preceding record.  The file must be
 on disk or tape.  Statement format:

     BACKSPACE ([UNIT=]u[,ERR=s][,IOSTAT=ios])
     BACKSPACE u

     u     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.
     s     Is the label of a statement that receives control 
           if an error occurs, prefaced by ERR=.
     ios   Is an 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 or append access.  Backspacing from record "n" can
 be done by rewinding to the start of the file and then performing
 n-1 successive reads to reach the previous record.  For direct and
 append access, the current record count ("n") is not available to
 the Fortran I/O system.