Default insertion

- allows the processor to insert missing attributes, elements, or chardata
- often allows briefer XML documents

Declaration:

defaults are declared separately from the structure definitions
defaultability of elements must be explicitly declared (Defaultable="yes")
cascading defaults can be specified in the application document

Insertion:

available defaults are guarded by boolean expressions
insertion is guided by the constraint checking - defaults are inserted on demand by the constraints
a notion of specificity determines the default when more than one is applicable

Default insertion is mixed into validation because of mutual dependence.