# ACL 2 seminar at U.T. Austin: Toward proof exchange in the Semantic Web

In our PAW and TAMI projects, we're making a lot of progress on the practical aspects of proof exchange: in PAW we're working out the nitty gritty details of making an HTTP client (proxy) and server that exchange proofs, and in TAMI, we're working on user interfaces for audit trails and justifications and on integration with a truth maintenance system.

It doesn't concern me too much that cwm does some crazy stuff when *finding* proofs; it's the proof *checker* that I expect to deploy as part of trusted computing bases and the proof language specification that I hope will complete the Semantic Web standards stack.

But N3 proof exchange is no longer a completely hypothetical problem; the first examples of interoperating with InferenceWeb (via a mapping to PML) and with Euler are working. So it's time to take a close look at the proof representation and the proof theory in more detail.

My trip to Austin for a research library symposium at the University of Texas gave me a chance to re-connect with Bob Boyer. A while back, I told him about RDF and asked him about Semantic Web logic issues and he showed me the proof checking part of McCune's Robbins Algebras Are Boolean result:

Proofs found by programs are always questionable. Our approach to this problem is to have the theorem prover construct a detailed proof object and have a very simple program (written in a high-level language) check that the proof object is correct. The proof checking program is simple enough that it can be scrutinized by humans, and formal verification is probably feasible.

In my Jan 2000 notes, that excerpt is followed by...

I offer a 500 brownie-point bounty to anybody who converts it to Java and converts the ()'s in the input format to <>'s.

5 points for perl. ;-)

Bob got me invited to the ACL2 seminar this week; in my presentation, Toward proof exchange in the Semantic Web. I reviewed a bit of Web Architecture and the standardization status of RDF, RDFS, OWL, and SPARQL as background to demonstrating that we're close to collecting that bounty. (Little did I know in 2000 that TimBL would pick up python so that I could avoid Java as well as perl ;-)

Matt Kauffman and company gave all sorts of great feedback on my presentation. I had to go back to the *Semantic Web Wave* diagram a few times to clarify the boundary between research and standardization:

- RDF is fully standardized/ratified

- turtle has the same expressive capability as RDF's XML syntax, but isn't fully ratified, and
- N3 goes beyond the standards in both syntax and expressiveness

One of the people there who knew about RDF and OWL and such really encouraged me to get N3/turtle done, since every time he does any Semantic Web advocacy, the RDF/XML syntax is a deal-killer. I tried to show them my work on a turtle bnf, but what I was looking for was in June mailing list discussion, not in my February bnf2turtle breadcrumbs item.

They asked what happens if an identifier is used before it appears in an **@forAll** directive and I had to admit that I could test what the software does if they wanted to, but I couldn't be sure whether that was by design or not; exactly how quantification and {}s interact in N3 is sort of an open issue, or at least something I'm not quite sure about.

Moore noticed that our conjunction introduction (CI) step doesn't result in a formula whose main connective is conjuction; the conjuction gets pushed inside the quantifiers. It's not wrong, but it's not traditional CI either.

I asked about ACL2's proof format, and they said what goes in an ACL2 "book" is not so much a proof as a sequence of lemmas and such, but Jared was working on Milawa, a simple proof checker that can be extended with new prooftechniques.

I started talking a little after 4pm; different people left at different times, but it wasn't until about 8 that Matt realized he was late for a squash game and headed out.

I went back to visit them in the U.T. tower the next day to follow up on ACL2/N3 connections and Milawa. Matt suggested a translation of N3 quantifiers and {}s into ACL2 that doesn't involve quotation. He offered to guide me as I fleshed it out, but I only got as far as installing lisp and ACL2; I was too tired to get into a coding fugue.

Jared not only gave me some essential installation clues, but for every technical topic I brought up, he printed out two papers showing different approaches. I sure hope I can find time to follow up on at least some of this stuff.

del.icio.us tags:Austin, semantic, web, logic, research

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