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John Graybeal <graybeal@m...> writes:
> If [W3C XML Schema can't require 1 a, 0 or 1 b, any number of c, in
> any order], can you offer a few words about why things were
> constrained in this way?
One reason was that there was disagreement about precedence -- that
is, does
( a & b? & c*)
mean
a c b c
is allowed, or only
a c c b
?
Another (perhaps, with hindsight, misguided) reason was to mitigate
the impact on parser writers.
And finally, there was perhaps a somewhat paternalistic (and therefore
again perhaps misguided) feeling that this is bad markup design - you
shouldn't want to do this. It makes documents hard to read, and hard
to process. Allowing something unique (e.g. your 'required_element')
to appear in the midst of a large collection of optional elements is
at best unhelpful.
It also seems likely to arise from a confusion between domain
modelling with document design -- if, as seems likely, order is not
significant (that is, where in the group a child occurs bears no
semantic weight), then good markup design is to choose an order and
require it.
Speaking only for myself,
ht
--
Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
Half-time member of W3C Team
2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
Fax: (44) 131 650-4587, e-mail: ht@i...
URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]
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