The other way to get inheritance of structure in Self is to _Define your prototypes as copies of the ones you want to extend:
... addSlotsIfAbsent ...
traits animal _Define: ( | p* = traits clonable. animal behavior | )
animal _Define: ( | p* = traits animal. animal data slots | )
traits mammal _Define: ( | p* = traits animal. mammal behavior | )
mammal _Define: animal copy _AddSlots: ( | p* = traits mammal. mammal data slots | )
Note the "animal copy _AddSlots:" this does the copy down when you read in the file.