OCL25 — OCL 2: what is a collection?

OCL 2 doesn't really define what a collection is. In essence,
a particular UML construct is arbitrarily designated as
the OCL collection, and a series of properties are assigned
to it

This question arises in 2 different ways:

• can you sub-class one of the concrete descendents in
  a UML diagram - by referring to the OCL package - and thereby
  add functionality to your own collection types

• are all parameterised classifiers collections? if you
  define a parameterised class, how does an OCL user or
  environment know whether it's a collection or not?

perhaps a stereotype should be intrroduced to allow for
unambiguous resolution of these issues