```
>>> class Foo:
... bar = 0
... @classmethod
... def set_bar(cls, v):
... cls.bar = v
...
>>> class Bar(Foo):
... ...
...
>>> Bar.set_bar(1)
>>> Bar.bar
1
>>> Foo.bar
0
>>> class Foo:
... bar = {}
... @classmethod
... def add(cls, k, v):
... cls.bar[k] = v
...
>>> class Bar(Foo):
... pass
...
>>> Bar.add(0, 1)
>>> Bar.bar
{0: 1}
>>> Foo.bar
{0: 1}
```
The collections are copied by reference into the descendants, whereas
_loaded is copied by value, so that one can stay. Before this patch,
every subtype was loading because _loaded was set per subclass, but they
were all registering with a common collection defined by UnitType rather
than their own class.
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