The XSLT layer's id() follows the XPath definition exactly: only
attributes the source's DTD declares as type ID identify elements
(internal or external subset, via the transform's resolver), and a
document without ID declarations yields the empty set. generate-id()
produces non-empty, distinct identifiers for attribute and namespace
nodes through their identity hash. Lookup implementations move to
XSLTLookups.swift for the file caps. Xalan baseline 131 -> 129.
Mihaela Mihaljevic
62db18f5f6