Frees the current record (that is, the last record read) in an
  indexed, relative, or sequential file.  By default, a record is
  locked when it is read.  The lock is normally held until your
  program performs another I/O operation on the unit (for example,
  rewriting the record, reading another record, or closing the file).

  Statement format:

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

     u    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    The label of a statement to which control is
          transferred if an error condition occurs.

     ios  A scalar default integer variable that is
          defined as a positive integer if an error occurs
          and zero if no error occurs.