Procedure SSubstr(var t: packed array [tlow..thigh: integer] of char;
                    s: packed array [slow..shigh: integer] of char;
                    start, span: integer);
   EXTERNAL;
  {*USER*
       -- String Package Module --
   The substring of string "s" defined by "start" and "span" is assigned
   to the target string "t".
  }