Listing:

    1  MODULE PList;
    2  TYPE Node = RECORD val : INTEGER; next : POINTER TO Node END;
    3  TYPE PNode = POINTER TO Node;
    4  VAR head : PNode; ExitCode : INTEGER;
    5  PROCEDURE Append(v : INTEGER);
    6  VAR q, r : PNode;
    7  BEGIN
    8    NEW(q);
    9    q^.val := v;
   10    q^.next := NIL;
   11    IF head = NIL THEN
   12      head := q
   13    ELSE
   14      r := head;
   15      WHILE r^.next # NIL DO
   16        r := r^.next
   17      END;
   18      r^.next := q
   19    END
   20  END Append;
   21  PROCEDURE Sum(p : PNode) : INTEGER;
   22  BEGIN
   23    IF p = NIL THEN
   24      RETURN 0
   25    ELSE
   26      RETURN p^.val + Sum(p^.next)
   27    END
   28  END Sum;
   29  BEGIN
   30    head := NIL;
   31    Append(5);
   32    Append(7);
   33    Append(9);
   34    ExitCode := Sum(head)
   35  END PList.

    0 errors