Arbitrary grouping

I'm new to XML schema and am confused on how I would create a schema that
introduces a set of arbitrary grouping for a set of elements.  For example:

	<x>
		<a/>
		<b/>
		<c/>
		<d/>
		<e/>
		<f/>
	</x>

or

	<x>
		<group>
			<a/>
			<b/>
		</group>
		<c/>
		<group>
			<d/>
			<e/>
			<f/>
		</group>
	</x>

or perhaps

	<x>
		<a/>
		<group>
			<b/>
			<c/>
			<d/>
		</group>
		<e/>
		<f/>
	</x>

would all be considered valid.  Additional constraints are that any number
of groups can occur, but the elements a through f can each only occur a
maximum of once inside the x element whether they are children of a group or
not.  

Peter Hunsberger