[plt-scheme] Lightweight object system

Hi,

I'm looking for a leightweight object system with all or some of the  
following features:

- preferably supports multiple inheritance
- "traditional" (i.e. not prototype-based or other less common  
approaches)
- preferably pure R5RS
- simple and small
- fast (might be used in a realtime scenario later)

I like ROOS and will perhaps use it, but it's PLT Scheme only at the  
moment. It might not be hard to port it to another dialect if needed,  
though.

What else would you recommend?

Best,

Erich