In section 3.8.3 this example is incorrect:

<form>
 {
    namespace a {"http://a.example.com" },
    attribute { xs:QName("a:id") } { "a-12-XE-45" },
    element { xs:QName("a:field")} { "Sample data" }
 }
</form>

The expression xs:QName("a:id") requires prefix "a" to be present in the static context, which it isn't; it's only present in the in-scope namespaces of the new form element.

It's also a poor choice of example because no computed namespace constructor is needed here; the namespace will be created automatically by the fixup process. A better example would be

<age xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> {
  namespace xs {"http://www.w3.org/2001/XMLSchema"},
  attribute xsi:type {"xs:integer"},
  23
}</age>

This illustrates the distinction between the xsi namespace which is created automatically because it's used in an attribute name, and the xs namespace which needs to be created by hand because it's used only in attribute content. (Though of course, it could be created statically if it's known statically).