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rationales for TEI extended-pointer keywords

From: Michael Sperberg-McQueen <U35395@UICVM.UIC.EDU>
Date: Tue, 10 Jun 97 19:00:56 CDT
Message-Id: <199706110050.UAA18150@www10.w3.org>
To: W3C SGML Working Group <w3c-sgml-wg@w3.org>
Some time ago, James Clark asked why the full array of tree-walking
keywords from the TEI extended-pointer notation was needed for XML
linking, and I was asked by the ERB to provide some account of why I
thought they were desirable.

I should note first that strictly speaking the 'functionality' of
extended pointers -- pointing at SGML elements or spans within SGML
documents -- is completely covered by the DESCENDANT keyword, with a
single numeric parameter.  DESCENDANT 42878 refers to the 42,878th
element in the document (the one with the 42,878th start-tag), and if we
limit ourselves to pointing at elements or pointing at spans that begin
and end with elements, everything else is syntactic sugar.  Byte offset
is also sufficient, as far as that goes -- in fact, byte offset is more
powerful than DESCENDANT because it handles sub-element addressing.

So no argument for *any* of the keywords can show that any of the
keywords adds new functionality; the TEI keywords that do, namely
PATTERN, TOKEN, STR, SPACE, FOREIGN, have all already been omitted.

On the other hand, no one is arguing for byte offset.  Why?  It's (a)
fragile, (b) opaque, and (c) given to silent failures.  We want
something better than byte offset or DESCENDANT with one argument
(which suffers from the same problems), because we want pointers that

  - are easy to understand / comprehensible / expressive
  - survive changes to the target document as well as possible
  - show signs of failure when they fail

All of the keywords in question (ID, CHILD, DESCENDANT, ANCESTOR,
PREVIOUS, NEXT, PRECEDING, FOLLOWING) serve substantially these same

When using natural language to point at texts, we routinely use typed
and untyped counts:  the third paragraph of the second chapter, or page
232.  Allowing restriction of the tree-traversal keywords by means of GI
and attribute-value pairs as well as counting means that the pointer is
clearer (the second quotation in German, or the second quotation after
the beginning of page 232, is clearer than "the 82305th sentence of the
text") and more robust:  a byte offset is broken if even one byte is
added or deleted before the target; a pointer to the second quotation
after the beginning of page 232 is broken only if a Q element is added
between the page boundary and the current second Q -- if the pointer
restricts by language as well, only a new quotation in German will break
the pointer.

No pointer is guaranteed safe forever when it's pointing at a changing
document.  But restriction by GI and attribute value, and being able to
traverse the tree in any desired direction, from any point chosen for
its relative stability (page 232 is *not* a good choice in this regard!)
provides much more robust pointers than counters without facilities for
such restrictions.

Counters by themselves are hopelessly fragile.

Why so many ways to walk the tree, though?  Well, primarily because in
general the best approach to pointing into changing read-only material
appears to be to point at some stable point and work from there.  The
stable point may be the end of a section, or the beginning; it may be an
element with an ID.  There is no good way to guarantee that the fixed
point will always be above the target resource in the tree, so CHILD and
DESCENDANT aren't enough; you need ANCESTOR.  It's not plausible to
assume that the fixed point is always to the left of the target --
sometimes it will be to the right, so if you have NEXT, you need PREVIOUS.

Some of the keywords are conservative (CHILD, NEXT, PREVIOUS) and are
useful for catching typos (a pointer to NEXT(82) is an error if there
aren't 82 younger siblings of the current location source); I suspect
that for stable documents it might be plausible to work exclusively with
these and ANCESTOR.

But if I want to point at the material on page 23 in a transcription of
Lachmann's edition which is currently being prepared, the most reliable
way to do it is to say "find the first <PB N='23' ED='La'> and go till
you reach the next <PB ED='La'>" -- it's more convenient, as well as
more robust, not to have to know exactly how many generations down in
the tree these milestone elements are.  A project I am working on
now is preparing documents in a data capture DTD and will translate
them automatically into an archival DTD later; at that time the nesting
level of virtually everything in the documents will change, since the
whole point of the data capture DTD is to have fewer levels and exploit
some regularities in the data.  I think it's a strong argument *for*
the TEI keywords that

  <xptr xml-link='simple'

will survive that transition, whereas the obvious choice using a
smaller set of keywords

  <xptr xml-link="simple"

is almost certain to break; if we add GIs to the parameters we will
hear about the breakage, but it will still break.

To make extended pointers robust, you need keywords that are *not*
limited to single-generation jumps or sibling searches:  you need
DESCENDANT, PRECEDING, and FOLLOWING.  (Note to those who don't want
to have to look this up:  descendant finds elements at any level of
containment; PRECEDING and FOLLOWING search left and right across the
entire tree -- unlike NEXT and PREVIOUS they are *not* limited to the
siblings of their location source.  Yes, I know, there's no way to
remember which is PREVIOUS and which is PRECEDING -- better names would
be welcome.)  Both functions are needed:  PREVIOUS for the case of
pointing into a relatively stable document where you *want* the more
restricted method of counting, to trap errors, and PRECEDING for being
able to cross container boundaries.

I had hoped to produce more concrete examples, but I am running late and
have to curtail this message.  So I'll leave it with the page-reference
example:  find, at any depth, the first PB with N=23 and ED=La, then
take everything up to the next PB from that edition.  Note that the next
PB is not guaranteed to have N=24:  page 24 could easily be blank and
not marked with a PB.  How do you do this reliably in a target document
subject to change without DESCENDANT, DITTO, and FOLLOWING?

I hope this helps clarify why I want a full set of tree-traversal
keywords.  Maybe James is right, and this is sort of a poor-folk's
query language.  If so, I suspect it's smaller and easier to implement
than any of the Rich Folk's query languages, and to a really
surprising degree it does get the job done.

-C. M. Sperberg-McQueen
 ACH / ACL / ALLC Text Encoding Initiative
 University of Illinois at Chicago

All opinions expressed in this note (except those I have quoted from
other authors) are mine.  They are not necessarily those of the Text
Encoding Initiative, its executive committee or other participants, its
sponsors, or its funders.  Ditto for the Model Editions Partnership
and the University of Illinois, and the XML Editorial Review Board.
Anyone who says otherwise is itching for a fight.
Received on Tuesday, 10 June 1997 20:50:21 UTC

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