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TEI-L  February 2003

TEI-L February 2003

Subject:

Pointers, links, representations

From:

Francois Lachance <[log in to unmask]>

Reply-To:

Francois Lachance <[log in to unmask]>

Date:

Sun, 23 Feb 2003 09:16:10 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (194 lines)

This is a rather lengthy post.  It serves as a close reading of 14.1.1
"Pointers and Links". Its aim is to approach the difference between
logical and semantic equivalency in alternative encodings possible under
the TEI Guidelines.

For those with the Green Book version of P3 at hand pages 396-397 cover
14.1.1 the section on Pointers and Links. I refer to this version of the
Guidelines because P4  does not replicate the description of the
permissable values for the "targOrder" and "evaluate" attributes which I
believe to be key in what follows.

The section begins with a cross-reference to the introduction of <ptr> and
<ref> [6.6  Simple Links and Cross references]. The guideline authors
announce the introduction of the <link> element. Some of this is pertinent
to the networks, graphs, trees discussion initiated recently with a thread
about the representation of vectors under TEI.  <ptr> and <ref> through
the values given to the "target" attribute form what could be called a
"starburst" pattern. The pointing or reference is to one or more
identifiable elements. The general pattern is from a distinct _here_, the
<ptr> or the <ref> to a set of _theres_. The set of _theres_ contains at
least one member and it can contain more than one. As the authors of the
Guidelines state the <link> element establish a different type of
relation. It "defines an association or hypertextual link among elements
[...]".  Its attribute is "targets". The difference between hypertextual
association and simple pointing is perhaps best characterized as the
difference between an insertion point and a cursor location in text
editing or wordprocessing environments. Or for those that inscribe
marginalia on printouts, a simple pointer can be a proofreading caret
inline indicating a missing letter; a hypertextual link could be a term
circled at the beginning of a paragraph and another towards the end of the
same rather long paragraph with a line joining them and perhaps a question
mark to indicate a suspicious shift in the semantic field. Suffice it to
say there are good grounds for distinguishing between the semantic
resonnance and the logical form of links and pointers. Unfortunately the
Guidelines seem to introduce an ambiguous usuage when the vocabulary of
"simple links" is applied to pointers.

[Aside <link> <ref> and <ptr> as members of the class pointer can take the
"crdate" attribute. In XML-ized TEI, the value of "crdate" cannot begin
with a numerical digit.  <ref crdate="MMDDYY 020803">Date</ref> formats
can of course be declared in the encoding description located in the
header.  It might be a useful practice to also indicate format in the
value of the attribute - thus prepping the way for the application of
attribute grammars with validation against XML schema and helping XSLT
processors pull/push data without axis skating.  End of the Aside.]

Version P3 of the Guidelines continue listing the common set of attributes
of the members of the class _pointer_. P3 also recapitulates the
permissible values for these attributes,  namely "targOrder" (Y | N | U)
and "evalutate" (all | one | none).

I like to think of the Guidelines as offering a problem set that the
reader/user needs to appropriate by working up examples on their own. In
my mind,  there is a workbook component to the Guidelines. P4 preserves
this wonderful aspect. It is like a hook:

[begin quote from P4 14.1.1]

<link type="echo" targType="p note" targets="p1 p2"/>

In this variation, not only must the link targets be either <p> or <note>
elements, but the one with identifier P1 must be a <p>, and that with
identifier P2 must be a <note>. Note that the present Guidelines provide
no direct way of saying that P1 may identify either a <seg> or a <p> and
P2 must identify a <note>.

[end quote from P4 14.1.1]

The Guidelines clearly state that there is "no direct way" which is of
course not equivalent to "no way". Reading the proposed example from P3,
one finds this example in proximity to the paragraph noting the lack of
provision of a direct way:

<link type="echo" targType="p seg note" targOrder="Y" targets="p1 p2"/>

This example exists in P4 (but earlier in the text). Between P3 and P4,
there has been a transposition of examples and hence a little loss in what
is to me a helpful hint found in the original sequencing of examples. In
any case let us take:

<link type="echo" targType="p seg note" targOrder="Y" targets="p1 p2"/>

and suggest that the depth of evaluation can be sued to do the work of
splitting which is preliminary to the work of linking.  The order is
important so that some encoding may be produced that finds a "way of
saying that P1 may identify either a <seg> or a <p> and P2 must identify a
<note>".

                        [P1 P2]  = targets
                        [p seg note] = target types
                        [echo] = type

 in order to produce this TEI fragment:

<list>
<link type="echo" targType="note" targets="P2" ID="LinkB"/>
<link type="echo" targType="seg p" targOrder="no" targets="P1 P2 LinkB"
evaluate="all" ID="LinkA"/>
</list>

which upon processing will return the following nodes:

Nodes from <link id="LinkA"/>
seg id=P1
seg id=P2
p  id=P1
p id=P2
link id=LinkB

Nodes from <link id="LinkB"/>
note id=P2

This I believe meets the requirement proposed in the example problem set.
that P1 may identify either a <seg> or a <p> and that P2 must identify a
<note>.

Further comments:

If <link id="LinkB"> were to target <link id="LinkA"> in the above
example, a potential loop would be created. This is an example where XML
Schema would prove very useful. As well using XSLT or Perl, an application
could locate all the instances of the string "LinkA" and return the values
of the ID attributes of the elements where those instances occur.

The value of the "evaluate" attribute is implied. An XSLT transformation
can be created so that if absent the value of "evaluate" is take to be the
default value which is "none".


Now then this lenghty examples leads up to the question of double
connection and continues to draw upon the distinction between _nodes_ and
_elements_:

[begin quote from P4 14.1.1]

Double connection among elements could also be expressed by a combination
of pointer elements, for example, two <ptr> elements, or one <ptr> element
and one <note> element. All that is required is that the value of the
target (or other pointing) attribute of the one be the value of the id
attribute of the other.  What the <link> element accomplishes is the
handling of double connection by means of a single element. Thus, in the
following encoding:

                      <ptr id="p1" target="p2"/> ... <ptr id="p2"
target="p1"/>

P1 points to P2, and P2 points to P1. This is logically equivalent to the
more compact encoding:

                      <link targets="p1 p2"/>

[end quote from P4 14.1.1]

Note that version P4 of the Guidelines introduces a case shift in the
values given to the ID attribtes and their reference in the surrounding
paragraphs. Version P3 of the Guidelines maintains consistent
XML-conformant reference (where "p1" and "P1" are distinct).

I am not sure under which logical regime the equivalency has been
established by the Guidelines. I am sure that semantically there is a
difference between the alternative encodings proposed.  The use of <link>
in this case can be represented as a double headed arrow between P1 and
P2. The use of two separate <ptr> elements can lead to a representation
where the pointing is modelled by one vector with a direction  and another
vector of equivalent magnitude but with a direction plus or minus 180
degrees of the other vector (if one is using Euclidean coordinates).

The representations admit different processing possibilities. A processor
could establish the position of the <link> element within the document and
calculate the magnitude and direction of vectors from the <link> element
to the targetted <ptr> elements. Researchers interested the genesis of
electronic text and the markers that establish the evidence for ways of
working with electronic text might in the future be interested in such
matters.

I hope that the forgoing is clear enough to indicate
1) my appreciation for the soundness of the Guidelines in all their
various versions
2) my abiding admiration for the working groups and for the editors who
have devoted much energy in ensuring the solid articulation of the tag
sets

Thank you

 --
<sigilla>
<civic.name>Fran&ccedil;ois
Lachance</civic.name> <self.desig>Scholar-at-large</self.desig>
<activity>Actively visiting <?insert URN?></activity>
<motto><w corresp="grok">gork</w> structure, savour <w
corresp="peace">content</w>, <s ana="play-with-piece">enjoy
form</s></motto>
</sigilla>

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