Hi Will

I am writing from ignorance about what all the words mean by now but I have some immediate queries about what you just wrote.

First, many thanks for having a go at explaining what this 'thingy' is.  I had not contributed to the discussion earlier as I was unsure (OK, I know I can look it up ...).

>>
At the level that Frank is taking about, he's discussing what  
different sorts of information can be used to format an arbitrary  
section heading. But the important thing to remember is that this is  
not the user interface.
>>
Are you sure about that?

>>
The idea of an "object type" in xtemplate refers only to the  
information given to it,
>>
It would be good if that were true but the impression I got from the discussion is that this thingy specifies more than this about the 'document interface' one of many 'user interface' around, I guess but probably what most people think of as 'the user-interace', of current latex at least. 

>>
not how that information eventually ends up  
in the typeset document.
>>
Agreed, but that is not usually thought of as interface, is it?

Of course the 'information supply' is an important part of the procesing that leads to the typeset document (or other current outputs from LaTeX code:-).
You cannot do anything (eg format) information that is not supplied and it must be suppied in suitable labelled containers too.

'No implementation without 'labelled information'  they cry!

OK, since I am here I will comment on the more substantive bits too.

>>
But it's important to design the "heading" object  
sufficiently generally that it can handle most sensible document  
designs.
>>
I am deeply unconvinced by this statement unless I interpret it as a 
truism, thus: 'sesnible document designs are precisely those that will be covered by this one thingy (is it really an object, in the normal informatics meaning of that word ... ask C Mittelbach:-).

There is no need whatsoever to have a 'one object fits all'.
>>
\section{Main Title That is Very Long and Involves Lots of Things}
   [
     label = main-title ,
     toc-entry = Main Title That is Very Long ,
     header-entry = Main Title ,
     numbered = false
   ]
>> 
that is fine as an example of a (classic LaTeX) document/user interface.  

I shall say more inanother post on a slightly different point about the structure (in the sense of abstract stuctured dosumants) and the nature of sectionhead-like elements.
 
>>
Internally, this might end up as the  equivalent of
>>

No, no, no!  Internally the syntax matters not at all in this context.
What you need to decide is at a higher level of abstraction what the structure of the information is (technically what is the content model for this subtree and which bits of information are values attached to nodes in the tree rather than the content of nodes).

But a comment on the syntax: internal things do not need to be locked away in what look here like 'tex parameters'.  They can be treated that way, of course, but whether they are or not should not influence the collection of information/content and building the internal sub-labelled-tree:
>>
\TheTemplateThatProcessesUnnumberedSections
   {Main Title That is Very Long and Involves Lots of Things}
   {main-title}
   {Main Title That is Very Long}
   {Main Title}
   {}{}{}% <- depending on any extra arguments for author info, etc.
>>

More soon, I hope, specifically on the information (and layout?) structure of 'sectioning and heads'.

Cheers,  chris 

(Trying to get a team going to implement (AFAP) Presentation MathML3 via LaTeX -- Standard or expl3 or anything!! -- to pdf for screen and paper rendering.)
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