BaseDecl and PrefixDecl?

Hello,

I am trying to figure out what semantic constraints are present on the
QuotedURI(Ref) for the BaseDecl and PrefixDecl productions.

Are the QuotedURIRefs for BaseDecl and/or PrefixDecl _required_ to be
absolute URIs?  May they be relative URIs?  Opaque?

Are there any constraints on the kind of URI that may be used for a
BaseDecl?  E.g., a URN?

Should the BaseDecl be used to resolve a relative URI in a PrefixDecl?

The spec says that a PrefixDecl may be re-bound.  However, I can not
think of any use case for this.  Since prefixes may only be bound at
the top of the query, and since a PrefixDecl may not use a QNAME, it
seems likely that re-defining a prefix will always indicate a user
error.

Is there any default namespace if none has been defined?  If not, is
it an error to write ":foo" if the default namespace has not been
bound?

Thanks,

-bryan