How are approxMatch matching rules declared?

[Hallvard B Furuseth <h.b.furuseth@usit.uio.no>]

[Models] 4.1.2 (Attribute Types) has no
    ["APPROX" SP oid]
part which can be used to declare the matching rule which
approxMatch filters on the attribute should use.  Should that be
fixed?  Or must an implementation which wants to use implement
approxMatch define a syntax like
    ["X-APPROX" SP oid]		; approxMatch matching rule
or do something like defining an implicit approx matching rule for
each syntax which supports approxMatch filters?
Should [Models] say something about this?

-- 
Hallvard