Item 25. Pretend There's No Such Thing as the PSVI
Schemas are an extremely useful means of checking preconditions on XML documents before processing them. While schema validation normally can't detect every possible violation of an application's constraints, it can often detect a lot of them. However, some schema languages (especially the W3C XML Schema Language) have a second, less salutary purpose. Although support is mostly experimental so far, the W3C XML Schema specifications indicate that a truly schema-aware parser should produce a post-schema validation information set (PSVI).
The concept is that the PSVI includes not only the actual content of the XML document but also annotations about that content from schemas. In particular, it ...
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