6. The method of claim 4 , wherein said one or more value constraints are based on an enumeration of values.
4. The method of claim 1 , wherein said generating said derived XML schema includes: making a second determination that among instances of a certain element contained in said collection of XML documents that at least a threshold portion of said instances conform to one or more value constraints; and in response to said second determination, causing said derived XML schema to define said one or more value constraints for said element.