Mandating element content

Hi,

    How to make the content of an element mandatory in XML Schema???
    For example:
    If I have an element "ELEM" of type xsd:string, then the valid values
for this element could be:
    1. <ELEM>some string</ELEM>
    2. <ELEM /> - I don't want this to happen in the instance document. I
want to mandate that there be some
        content for this element. How do I do this???

Avinash Subramanya
Developer
Aztec Software And Technology Services Limited
23, 3rd 'A' Cross, 18th Main, 6th Block, Koramangala
Bangalore - 560095, India
Phone: 5522892/93 Ext: 205
Fax: 91-80-5521987
Mobile: 98441 29329