Qualifiers, written in brackets after the path name of a feature call,
can express two different things.

• qualifying use: A qualifier is used to give the qualifing value of
  a qualified association (chapter 7.5.7).
• navigational use: A qualifier is used to refine the navigation to
  association classes. While this navigational use is necessary
  only with recursive associations, it is legal for every navigation
  to an association class (chapter 7.5.5).

There is no way to distinguish these two sorts of qualifiers. There are
even expressions where both uses of the qualifiers would be necessary at
once, but this problem is restricted to such models that contain a
recursive, qualified association that has an association class.

Example where navigational and qualifing use cannot be distinguished:

There are two classes "Bank" and "Person", with a association between
them qualified by the account number (an Integer). The association end
at the class Person is named "customers".
An additional class "Company" has an attribute "customers" of type
Integer.

Now consider the subexpression "bank.person[customers]" in the context
of Company. "bank" clearly is a navigational expression. But "customers"
could either mean the attribute of Company, since Company is the context
of the expression (that is qualifying use as defined in 7.5.7); or
"customer" could mean the name of the association end (navigational use
as defined in 7.5.5). In the first case, the result type would be
Person, in the second case Set(Person).

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

OrderedSet isn't discussed in the section on semantics