An updated version should appear soon.  I have made the following changes:

   - I have dropped the mutability/immutability distinction
   - I have removed subtyping from define-record-type and added
     define-record-scheme for encoding polymorphism.  Type schemes
     are similar to Java interfaces or Haskell typeclasses.
   - Minor changes in syntax of record-compose.
   - I have based the implementation on SRFI-9 records.
   - Using a technique borrowed from Taylor Campbell's syntax-rules
     tutorial, which I dub meta-quasiquotation, I was able to
     dramatically simplify the implementation and dramatically
     increase its readability.  It should now be almost trivial to
     port to your favourite low-level macro system.

Andre