Program Repair Shop

Encapsulate common operations

        $a1->add_observer($g1);
        $a1->add_observer($g1);

I don't know what the "right" thing is here
- Particularly in conjunction with deletions, etc.
- So I'll just try to get it to be consistent:
  - Each unique observer is notified exactly once
This will come in handy:

        sub _uniq {
          my %seen;
          return grep !$seen{$_}++, @_;
        }