On the subject of Heyting algebras, usage seems to be ambiguous as to whether they should have (and their morphisms preserve) finite joins. I suggest that we should say "Heyting lattice" if they should, and "Heyting semilattice" if not. More generally, Vaughan said,
Nowadays when I hear "Never heard of x" my subconscious seems to turn it into "never heard of Wikipedia."
I too turn to Wikipedia for information on most subjects. For example its medical information is far superior to any other lay source that I have seen. But I have two reservations: Authority. Journalists like to take swipes at it on the grounds that anyone can edit it, but in my opinion they over-estimate the reliability of "authoritative" sources. A traditional paper encyclopedia consults only a small number of experts on each topic, so it's likely to be cliquey. On the other hand, there are frequently stories in www.TheRegister.co.uk (online geek news) about cliques taking over Wikipedia. Closer to home, the coverage of mathematics is extremely poor in comparison to other subjects. Usually, there is just the stark classical undergraduate definition, with neither advanced mainstream material nor any constructive critique. In my work on ASD, particularly its application to real analysis, I have wanted to refer to classical sources as a background, but on none of the relevant topics have I considered the Wikipedia article to be anywhere near satisfactory. All spaces are Hausdorff, and Excluded Middle is a Fact. I have thought about rewriting the articles on Dedekind cuts, locally compact spaces and some other things, but am afraid that my contributions will just be "reverted". Maybe if other categorists and constructivists joined in too, I would feel in better company. No, I don't want knock Wikipedia. It's a Good Thing, in principle. And I would like to encourage others to improve the mathematical coverage. By the way, there's also PlanetMath.org, in which authors "own" their articles, unless they have demonstrably abandoned them. Since I'm here, I would like to point out that there are thoroughly revised versions of The Dedekind Reals in ASD (with Andrej Bauer) and A Lambda Calculus for Real Analysis on my web page at www.PaulTaylor.EU/ASD/analysis.php The second of these contains a "need to know" introduction to the Scott topology, proof theory and the lambda calculus, ie it is written with the general mathematical audience in mind. Paul Taylor