[xacml] function semantics

I would like to get some clarification on function 
semantics.

 

For example, if I want to compare 2 string values 
in the context for equality

I'd like to use 'string-equal' function with 2 
designator arguments.

 

Designators select a sequence of values from the 
context, but arguments to

the 'string-equal' take single-valued string. (see 
function table spreadsheet).

 

So simple syntax: 'string-equal subj-attr-desig 
subj-attr-desig' is not legal.

How is it supposed work?

 

The other question I have is about sets and 
sequences. They are used

throughout function table, and I'm not sure what is 
difference.

 

Simon