Logged In: YES 
user_id=764593

There is an implicit assumption that any component/part with 
its own 'components' attribute will be another UserString (or, 
at the very least, will act like one when traversed).

Would it be better to check whether the subobject actually is 
a UserString (since new-style objects can freely use multiple 
inheritance), or to do some sort of guard in add?  (Perhaps 
immediately stringifying anything with a components attribute 
that is not a UserString?)