This seems like a sensible addition to me. I was thinking to myself "this could be nicely expressed as a Pragma", then I realised of course that Pragma is only in XQuery.

Let's leave this open for a couple of days for any more comments, and then all being well, let's merge it...

Do we also want to provide support for declaring the default element namespace (something like with prefix := ... or with prefix '' := ...)?

And I guess that new prefixes will shadow existing ones?

So let's try and flesh out the proposal.

We have a new kind of PrimaryExpr:

[56] PrimaryExpr ::= .... | WithPrefixExpr
[56a] WithPrefixExpr ::= "with" "prefix" NCName? "=" URILiteral ("," "prefix" NCName? "=" URILiteral)* ) EnclosedExpr

Semantics: The result of a WithPrefixExpr is the result of the EnclosedExpr, evaluated with a static context that varies the set of namespace bindings.

Specifically, for each "prefix" clause, the static context for EnclosedExpr (the "inner context) is based on the static context of the WithPrefixExp (the "outer context") altered as follows:

• If an NCName is present, then:

** if the URILiteral is a zero-length string, any binding of the NCName to a URI in the outer context will be absent from the inner context (if there is no such binding in the outer context, then the declaration has no effect)

** Otherwise, the set of statically known namespaces in the inner context contains a (prefix, URI) binding for this NCName and the corresponding URILiteral, replacing any binding for this NCName that exists in the outer context

• If the NCName is absent, then the default element/type namespace is set to the value of the URILiteral, or to absent if the URILiteral is the zero-length string.

Error conditions:

• The same NCName must not appear more than once.

• There must not be more than one "prefix" clause with an absent NCName.

Note:

• the syntax is designed with extensibility in mind, so that we could add other clauses that vary the static context, for example "with default-collation = URILiteral {...}"

• I've used "=" rather than ":=" for consistency with "declare namespace" in XQuery.