More subquery questions - BASE & PREFIX

(Can you tell that I'm gearing up for that implementation I promised?)

The following questions are mainly for existing implementors of 
subquery, though if you have a strong opinion on how these points should 
be specified, I'd be pleased to hear them.

Do existing subquery implementations allow subqueries to define its own 
base uri? (i.e. include a BASE clause) Do the subqueries inherit the 
BASE of the parent?

Similarly for PREFIX, can a subquery define its own prefixes? do they 
override prefixes in the parent query? is it additive? (i.e. if I have 
PREFIX p1: <...> in the parent and PREFIX p2: <...> in the subquery, can 
I use both p1: and p2: throughout the subquery?)

Lee