CONTENTS Introduction ................................................................................................5 §1. The purely category theoretical version of Birkhoff's theorem................7 §2. Category theoretical study of generalized identities............................19 §3. Generalized identities in partial algebras.............................................29 §4. Calculus...............................................................................................41 §5. Examples.............................................................................................47 §6. Some model-theoretic consequences.................................................48 References...............................................................................................50
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