Two representations of the semantics of TEI Lite

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Two representations of the
semantics of TEI Lite
Sperberg-McQueen, C. M.
cmsmcq@blackmesatech.com
Black Mesa Technologies LLC, USA
Marcoux, Yves
yves.marcoux@umontreal.ca
Université de Montréal, Canada
Huitfeldt, Claus
Claus.Huitfeldt@uib.no
Department of Philosophy, University of
Bergen
Markup languages based on SGML and
XML provide reasonably fine control over
the syntax of markup used in documents.
Schema languages (DTDs, Relax NG, XSD, etc.)
provide mature, well understood mechanisms
for specifying markup syntax which support
validation, syntax-directed editing, and in some
cases query optimization. We possess a much
poorer set of tools for specifying the
meaning
of the markup in a vocabulary, and virtually
no tools which could systematically exploit any
semantic specification. Some observers claim,
indeed, that XML and SGML are “just syntax”,
and that SGML/XML markup has no systematic
semantics at all. Drawing on earlier work
(Marcoux et al., 2009), this paper presents
two alternative and complementary approaches
to the formal representation of the semantics
of TEI Lite:
Intertextual semantics
(IS) and
Formal tag-set descriptions
(FTSD).
RDF and Topic Maps may appear to address
this problem (they are after all specifications for
expressing “semantic relations,” and they both
have XML transfer syntaxes), but in reality their
focus is on generic semantics — propositions
about the real world — and not the semantics of
markup languages.
In practice, the semantics of markup is
most of the time specified only through
human-readable documentation. Most existing
colloquial markup languages are documented
in prose, sometimes systematically and in
detail, sometimes very sketchily. Often, written
documentation is supplemented or replaced
in practice by executable code: users will
understand a given vocabulary (e.g., HTML,
RSS, or the Atom syndication format) in terms
of the behavior of software which supports or
uses that vocabulary; the documentation for
Docbook elevates this almost to a principle,
consistently speaking not of the meaning of
particular constructs, but of the “processing
expectations” licensed by those constructs.
Yet a formal description of the semantics of a
markup language can bring several benefits. One
of them is the ability to develop provably correct
mappings (conversions, translations) from one
markup language to another. A second one is
the possibility of automatically deriving facts
from documents, and feeding them into various
inferencing or reasoning systems. A third one
is the possibility of automatically computing
the semantics of part or whole of a document
and presenting it to humans in an appropriate
form to make the meaning of the document (or
passage) precise and explicit.
There have been a few proposals for formal
approaches to the specification of markup
semantics. Two of them are
Intertextual
Semantic Specifications
, and
Formal Tagset
Descriptions
.
Intertextual semantics (IS) (Marcoux, 2006;
Marcoux & Rizkallah, 2009) is a proposal to
describe the meaning of markup constructs
in natural language, by supplying an IS
specification (ISS), which consists in a pre-text
(or text-before) and a post-text (or text-after)
for each element type in the vocabulary. When
the vocabulary is used correctly, the contents
of each element combine with the pre- and
post-text to form a coherent natural-language
text representing, to the desired level of detail,
the information conveyed by the document.
Although based on natural language, IS differs
from the usual prose-documentation approach
by the fact that the meaning of a construct
is dynamically assembled and can be read
sequentially, without the need to go back and
forth between the documentation and the actual
document.
Formal tag-set descriptions (FTSD) (Sperberg-
McQueen et al., 2000) (Sperberg-McQueen &
Miller, 2004) attempt to capture the meaning
of markup constructs by means of “skeleton
sentences”: expressions in an arbitrary notation

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into which values from the document are
inserted at locations indicated by blanks. FTSDs
can, like ISSs, formulate the skeleton sentences
in natural language prose. In that case, the main
difference between FTSD and ISS is that an
IS specification for an element is equivalent
to a skeleton sentence with a single blank, to
be filled in with the content of the element.
In the general case, skeleton sentences in an
FTSD can have multiple blanks, to be filled in
with data selected from arbitrary locations in
the document (Marcoux et al., 2009). It is more
usual, however, for FTSDs to formulate their
skeleton sentences in some logic notation: e.g.,
first-order predicate calculus or some subset of
it.
Three other approaches, though not directly
aimed at specifying markup semantics, use
RDF to express document structure or
some
document semantics, and could probably be
adapted or extended to serve as markup
semantics specification formalisms. They are
RDF Textual Encoding Framework
(RDFTef)
(Tummarello et al., 2005) (Tummarello et al.,
2006), EARMARK (
Extreme Annotational RDF
Markup
) (Di Iorio et al., 2009), and GRDDL
(
Gleaning Resource Descriptions from Dialects
of Languages
) (Connolly, 2007).
RDFTef and EARMARK both use RDF to
represent complex text encoding. One of their
key features is the ability to deal with non-
hierarchical, overlapping structures. GRDDL
is a method for trying to make parts of the
meaning of documents explicit by means of
an XSLT translation which transforms the
document in question into a set of RDF
triples. GRDDL is typically thought of as a
method of extracting meaning from the markup
and/or content in a particular document or
set of documents, rather than as a method
of specifying the meaning of a vocabulary;
it is often deployed for HTML documents,
where the information of most immediate
concern is not the semantics of the HTML
vocabulary in general, but the implications of
the particular conventions used in a single
document. However, there is no reason in
principle that GRDDL could not be used to
specify the meaning of a markup vocabulary
apart from any additional conventions adopted
in the use of that vocabulary by a given project
or in a given document.
If proposals for formal semantics of markup are
scarce, their application to colloquial markup
vocabularies are even scarcer. Most examples
found in the literature are toy examples. A
larger-scale implementation of RDFTef for a
subset of the TEI has been realized by Kepler
(Kepler, 2005). However, as far as we know,
no complete formal semantics has ever been
defined for a real-life and commonly used
colloquial vocabulary. This paper reports on
experiments in applying ISSs and FTSDs to
an existing and widely-used colloquial markup
vocabulary: TEI Lite.
Developing an ISS and an FTSD in parallel for
the same vocabulary is interesting for at least
two reasons. First, it is an opportunity to verify
the intuition expressed in Marcoux et al., 2009
that working out ISSs and FTSDs involves much
the same type of intellectual effort. Second, it
can give insight into the relative merits and
challenges of natural-language vs logic-based
approaches to semantics specification.
The full paper will focus on the technical and
substantive challenges encountered along the
way and will describe the solutions adopted.
An example of a challenge is the fact that TEI
Lite documents can be either autonomous or
transcriptions of existing exemplars. Both cases
are treated with the same markup vocabulary,
but ultimately, the meaning of the markup is
quite different: in one case, it licences inferences
about the marked-up document itself, while
in the other, it licences inferences about the
exemplar. The work reported in Sperberg-
McQueen et al., 2009 on the formal nature
of transcription is useful here to decide how
to represent statements about the exemplar,
when it exists. However, the problems of
determining whether any particular document is
a transcription or not, and of putting that fact
into action in the generation of the semantics
remain. One possible solution is to consider as
external knowledge the fact that the document is
a transcription. In the FTSD case, that external
knowledge would be represented as a formal
statement that could then trigger inferences
about an exemplar. In the ISS case, it would
show up as a preamble in the pre-text of
the document element. Another solution is
to consider the transcription and autonomous
cases as two different application contexts
of the vocabulary, and define two different