That's discussed in this paper: CommonLoops: merging Lisp and object-oriented programming https://dl.acm.org/doi/pdf/10.1145/28697.28700 https://dl.acm.org/doi/

That's discussed in this paper: CommonLoops: merging Lisp and object-oriented programming https://dl.acm.org/doi/pdf/10.1145/28697.28700 https://dl.acm.org/doi/pdf/10.1145/28697.28700 CommonLoops was proposed by Xerox to be the OOP system of Common Lisp in the standardization process. It was then decided to design a new system called Common Lisp Object System (CLOS), starting with a merge of the features of New Flavors (MIT/Symbolics) and CommonLoops (Xerox). Xerox implemented CLOS by modifying its CommonLoops implementation, during the standardization process. Thus Portable CommonLoops (PCL) was eventually the prototype CLOS + MOP implementation.