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It is possible to use xs:choice element inside xs:sequence,
<xs:sequence>
<xs:choice>...</xs:choice>
</xs:sequence>
but not inside xs:all. Why is that? Does that create some ambiguity when parsing xml?
This can only really be answered by the designers of XML Schema. You could pose it to the XSD newsgroup (xmlschema-dev@w3.org). My guess is that ambiguity and/or complexity are the problem.
For example of an ambiguity, consider having an all group with two children that are choice groups and each of those has an element named ‘X’, of two different types. When an X element is encountered by the parser, which X is it: the one belonging to the first choice group or to the second choice group (note that it does matter, because you must know which type to validate it with)? Perhaps the designers could have specified constraints to avoid such ambiguities, but perhaps doing that was just too complex and not worthwhile.