Assign procedure

  DECLARATION:  Assign(var F; Name : String);

      PURPOSE:  Assigns the name of an external file to a file variable

         UNIT:  System

      REMARKS:  Where F is a file variable of any file type, and Name is a
                string-type expression.  Once assigned, all operations on F
                will be performed on the external file with the filename Name.

                The Filename can be any valid DOS filename with or without a
                Drive identifier and/or directory path.  When no path is
                included, the current directory is assumed.  The maximum
                length of any file name, including the drive and path if
                included, is 79 characters.

                If Assign is given a filename of 0 characters ( '' ) then
                the file variable will be associated with the standard input
                or output device.  This enables the use of DOS redirection
                procedures.

 RESTRICTIONS:  Assign cannot be performed on an open file.