Draft new Additional Document Constraints

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

From Section 4 of the hub document, titled Additional Document Constraints

  In addition to the UBL 2.1 document constraints formally expressed by
  the schemas in Section 3, "UBL 2.1 Schemas", UBL mandates several
  other rules governing conformant UBL 2.1 instances that cannot be
  expressed using W3C Schema.These additional UBL document rules,
  addressing instance validation, character encoding, and empty elements,
  are specified below.

During the TC call earlier today, it was suggested that a couple new 
constraints be added:

    http://lists.oasis-open.org/archives/ubl/201211/msg00038.html

Looking back at NDRs as far as December 2009 there appear to have 
only been up to 6 "IND" rules, so these two new ones are numbered 
"IND7" and "IND8".

Below is the first draft of these new rules to be considered for the 
hub document.  Please post your comments and suggestions.  My worry 
is that this rule is too general and may inadvertently trip on some 
elements that are meant to be expressed with multiple element 
instances (though I thought that from a modelling perspective this is 
obviated by the technique we used for "Address Line. Line").

I've tried to be brief, but sufficiently descriptive, in the 
background for the rules and the expression of the rules.  Feel free 
to suggest any alternatives.

. . . . . . . . Ken

4.4 Natural language text elements

Throughout the UBL document model there are a number of natural 
language text elements, such as "Note" and "Description", in which 
prose information is expressed.  Such elements are of type Text, 
though not exclusively as there are other Text fields that are not 
for natural language text, such as "Address Line. Line".  Natural 
language text elements are repeatable.  For example, the Note element 
is repeatable and all sibling Note elements are adjacent and are 
under a common parent element.

The repeatable nature of natural language is not intended to capture 
the prose concept of paragraphs or any other kind of 
compartmentalized subset of content.  Rather, the repeatable nature 
is solely intended to capture the same information in different 
natural languages.  For example, an English description and a French 
description distinguish exactly two sibling Description elements, 
even if either or both descriptions are comprised of multiple paragraphs.

UBL prohibits ambiguous expression of sibling natural language 
elements by imposing the following constraints:

  [IND7] Where two or more sibling Text elements of the same name exist in
         a document, no two can have the same languageID attribute value.

  [IND8] Where two or more sibling Text elements of the same name exist in
         a document, no two can have the languageID attribute absent.

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