----------
| Session |
| object  |
----------
|obj ref: |
| $store  |     -----------------
|     \___|____\| Bridge::Simple |             | Database|
|         |    /|                |             | objects:|
interface:      | $map    $obj --------------->| DB_File,|
 fetch()         ---:------------              | SQLite  |
 fetch_ids()        :
 store()            :
 delete()           :
                    :
               -----------
               (maps the interface
                of the using object
                to the implementation
                interface, eg, the DB_File
                implementation)
                {
                   fetch=>'get_value',
                   fetch_ids=>'get_keys',
                   store=>'set_value',
                   delete=>'delete',
                }