(require 'oop) or
`Yet Another Scheme Object System' is a simple object system for Scheme
based on the paper by Norman Adams and Jonathan Rees: Object
Oriented Programming in Scheme, Proceedings of the 1988 ACM Conference
on LISP and Functional Programming, July 1988 [ACM #552880].
Another reference is:
Scheming with Objects
AI Expert Volume 7, Number 10 (October 1992), pp. 24-33.