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jcowan@reutershealth.com wrote:
>Max Chappell scripsit:
>
>
>
>>Taking the example you gave -
>>You can't say if <ParentNode> has @flavour='B' Then only allow child
>>elements of type <B>.
>>
>>Tall order I know, but something I come up against on a daily basis.
>>
>>
>
>Consider using RELAX NG, then, in which such things are easily accomplished
>using fixed-value attributes and a choice.
>
>
Or Schematron:
<rule context="ParentNode[@flavour='B']">
<assert test="count(B) = count(*)">
A ParentNode with flavour attribute of B only can have children of
type B
</assert>
</rule>
Even RELAX NG, with enormous respect to it, only allows very simple, linear
context constraints: it is a lot more idiomatic than WXS (allowing you
to treat
the element name and attribute values as the same kind of thing) but still
all the current grammar systems share the same intrinsic shortcoming:
there is no reason to expect that an arbitrary database will naturally form
a tree that has a grammar that expresses all the relations. So grammars
force you to use use some other layer (with different concepts) for the
"tricky"
(embarrassing?) relations: ID/IDREF, key/keyref, external links etc etc.
If your data is such that you has occurrence constraints that
cannot be expressed by XPaths using the ancestor, attribute
and previous-sibling axes, then you should consider using
some path-based schema language such as Schematron.
Cheers
Rick Jelliffe
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