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An [XML] document is WellFormed if * all tags are closed * they nest correctly * all attribute values are quoted * no invalid characters appear * and a few more minor criteria are met. This is not valid: __<p>__A paragraph __<em><strong>__here__</em></strong>__. __<p>__And another one there. The paragraph tags are not closed, and the nesting for the emphasis and strong tags is wrong. To be WellFormed, this piece of the document has to be written like so: __<p>__A paragraph __<strong><em>__here__</em></strong>__.__</p>__ __<p>__And another one there.__</p>__ In the second fragment, neither the __<b>__ nor the __<c>__ tag are closed. Unlike [SGML], [XML] does not allow tags to be automatically closed when the enclosing tag is closed. This is the reason why the __<p>__ tag in [HTML]/[XHTML] gives people grief -- in [HTML] you only need to put in the opened tags while in [XHTML] you need to put in both the opening and the closing tag. There is no way to directly express non-nesting overlapping ranges in [XML]. While [SGML]'s [CONCUR] feature is a solution, it appears never to have been implemented, and has not been included in [XML]. If you need such, a way to express it might be something like __<foo type="a" partof="1">__ ... __</foo>__ __<foo type="a" partof="1" partof="2">__ ... __</foo>__ __<foo type="b" partof="1">__ ... __</foo>__ If you are certain that this is too bulky (especially when you know you have a very large number of overlapping structures), a flat alternative might look like __<a id="1"/>__ ... __<b id="2"/>__ ... __<a id="1"/>__ ... __<b id="2"/>__ You can reconstruct the tags as necessary from either form. The flattened form allows to apply some kinds of diff-like reasoning which are much more difficult on trees, but the structured form is generally easier to machine process, eg using [XSLT]. WellFormed [XML] differs from [Valid] [XML] in that [Valid] [XML] is not only WellFormed, but also has been (or could be) checked against a [Schema] or [DTD].
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