suggested paragraph for 2.2 about alternatives

Hi all, I noticed our current section 2.2 lacks any text about how
multiple schema mappings are alternatives. Here's a proposed paragraph
to be added after the paragraph that starts with "The value of the
loweringSchemaMapping attribute is a set of zero or more":

        Both liftingSchemaMapping and loweringSchemaMapping attributes
        are sets with potentially multiple values. Multiple schema
        mappings are to be treated as alternatives, i.e. the client
        processor should choose one of them to apply, and the choice is
        fully at the client processor's discretion. For example, a
        mapping can be selected based on what mapping language the
        processor supports (different alternatives can use different
        languages), based on the availability of the mapping document,
        or by other preferences.

This is, of course, open to rewording to make it sound clearer and to
make it fit the spec. 8-)

Hope it helps,

Jacek