The compiler implements simplification 4.9-4.10 (ns inheritance across
ancestors and document boundaries; QName resolution against in-scope
xmlns declarations; the attribute name= shorthand stays namespace-less),
include brings in the included grammar's start with overrides (4.7),
multiple starts and defines combine with a single omitted combine
merging by the named method (4.17), data/except lands as a dataExcept
pattern in the derivative engine (4.12), an empty element matches data
whose lexical space admits the empty string, and whitespace-only
attribute values match empty-matching patterns. Spec suite: valid
274/289, invalid 279/291.
