MDA - markup/xml/validation/schematron

Comment:: phases (top-level) -- patterns -- rules (define the context) -- assertions
advantage: VERY modular
advantage: rather intuitive -- prose english constraints can be readily translated into rules
advantage: no problem with namespaces and versioning
Comment:: rule-based (declarative) model -- not grammar (constructive)
advantage: rather powerful
Comment:: uses XPath to define patterns where constraints should apply
Comment:: Structure Validation Language using Patterns in Trees
Comment:: ISO Schematron -- used as one of the validation engines in DSDL
advantage: can be implemented via XSLT

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