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For four years in the late 1990's and early 2000's I worked at Stanford University’s Section on Medical Informatics doing research in Artificial Intelligence. I was one of the primary architects on the Protege project (an open-sourced knowledge representation system) and spent quite a bit of time thinking about how to represent knowledge, the logical structure of knowledge, how to define constraints on information, and how to classify algorithms (a.k.a. “problem-solving methods”).
This talk, from 2001, describes the underlying architecture formal knowledge model used in Protege, how "slot widgets" play in the system, and goes on to describe PAL: the Protege Axiom Language. It's long, and really only for knowledge representation afficionados, but it's pretty complete.