[xacml] [CR] Alternative for AttributeValue change proposal

>2. Adding a separate construct to XACML policy schema to represent
>sequences of attribute values, such that you cannot create sequeences of
>sequences.

As an alternative to my proposal to modify schema element <attributevalue>,
I propose to define standard core function collection

<type>-sequence

of undefined arity, that takes arguments of singleton <type> kind and return
a sequence<type> value,  a sequence of its argument values.

In this case, when a literal sequence definition is needed, this funciton
can be used.  No schema modification is needed.

The drawback is that it can not be used to specify an argument of
sequence<type> kind for predicate functions that can be used in <Match>
elements.

Daniel.