IDREFS types in schemas

Hello, 

I have a short question... I have the following lines in my .xsd file : 

<xs:attribute name="inp" type="xs:IDREF" use="required"/>

<xs:attribute name="corresplist" type="xs:IDREFS"/>

I also define an element GInP, and an element Correspondance. I just want to
specify that my attribute 'inp' is a GInP and that the IDREFS of
'corresplist' refer to Correspondance (s)...

Is it possible ? And if it is, what's the trick ??

Thanks a lot !

Morgane.