at least one

Sorry to bother you again,
I try to look around but I can't find a solution.
I would like to know if there is some way in XSD to say that a complex
element should be composed by any but at least one element of a group(=
some) of elements.
For example:
the group is :
	<high></high>
	<medium></medium>
	<low></low>

This is CORRECT:
<filters>
	<high></high>
	<medium></medium>
	<low></low>
</filters>

This is CORRECT:
<filters>
	<high></high>
	<low></low>
</filters>

This is INCORRECT:
<filters>

</filters>
I know the maximum number of elements of the group but I don't know
which and how many they could choose. Everything it's ok but there
should be at least one!
Thank you for you help
Debora Vanni