XML schema : refer problem

[Geert Delanote <geert.delanote@gmail.com>]

Suppose I have the following document :
<?xml version="1.0" encoding="UTF-8"?>
<organisation>
	<person id="1">
		<poll id="a">
			<answer id="1"/>
			<answer id="3"/>
		</poll>
	</person>

	<poll id="a">
		<question id="1"/>
		<question id="2"/>
	</poll>
	<poll id="b">
		<question id="1"/>
		<question id="3"/>
	</poll>
</organisation>

In my schema definition i want the following constraints :
- a poll has a unique id within an organisation : key on id in
organisation scope : No Problem
- a question has a unique id within a poll : key on id in poll scope :
No Problem
- a person can only answer a defined poll : keyref to the key of poll
: No problem
- a person can only answer questions of the referenced poll : ...
problem ... How do I do that? The combination poll-id question-id is
unique but I can't define a key on that combination which I need to
refer to.

Extra question : how can I ensure that the person answers every question?
The question element contains lots of information that I don't want to
duplicate...