TinyCLOS is cool: it's an implementation of OO (ie MOP), but doesn't limit slots (aka object properties) to symbols! https://community.schemewiki.org/?Tiny-CLOS
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The book 'The Art of the Metaobject Protocol' has two chapters in the public domain and available online!
Chapter 5: Concepts
Chapter 6: Generic Functions and Methods
Implementing a subset of CLOS for performance, forgoing MOP as a merely development-time feature: http://t.co/qK0GAjyxrt