+---------+        +---------+        +---------+
   |  Filter |        |   Safe  |        |   Base  | 
   +---------+        +---------+        +---------+
        |                  |                  |
        +------------------+------------------+
                           v
                    +-------------+
                    | Safe_Filter | @ISA=qw(Filter Safe Base);
                    +-------------+