| | Practical Foundations of Mathematics (Site not responding. Last check: 2007-11-04) |
 | | HSL of Boolean algebras in the category of Heyting semilattices has both a reflection and a co-reflection, and these functors are the same. |
 | | Finally, elementary sketches present equational many-sorted unary theories, and the classifying category is free on the sketch. |
 | | If we take the semantic option, then the universal property of the classifying category is more complicated than Definition 7.1.1: the interpretation functor [[-]] is only unique up to unique isomorphism - if it is defined at all, as some Choice is to be made. |
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