Identifying absent behavior

Hello All,

I'm working on a Schema that need's to know if the value of an XML mark up
is absent.
If there are any absent values it must not validate the file. I don't know
if the W3C has made a decision on the  method yet.  If anyone knows how to
deal with this please tell me.

	Absent c element: <t></t>
	Empty  c element: <t><c/></t>
	Nil    c element: <t><c xsi:nil="true"/></t>

Thanks so much in advance.
Garrett