index and ADT

Hi,
I would like to index a table which contains a column with an ADT type. for instance :

Create type rectangle_t as object ( width INTEGER, Longr INTEGER);

Create table rectangle (figure rectangle_t);

I would like to index my "rectangle" table on the "figure" column. I have tried :

create index on rectangle(figure);

but that doesn't work ! I have the Error message : ORA-02327: cannot create index on column with datatype ADT

Any help would be appreciated !
Thanks in advance

                                Didier