how to write xsd validation checking one tag value remaing tags should be there and not null

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ASP.NET

Hi I have xml like below .I want to write xsd validation in sucha a way that if usepasfro allocation tag contains 1 as its value then bleow tags are manadatory means that shouuld be there in xml or if the value is 0 then no need

</UsePasforAllocation>1<UsePasForAllocation>
            <PASPrefix>Awe1</PASPrefix>
            <PASBusinessStartDate>06/06/2011</PASBusinessStartDate>
            <PASContribution>3</PASContribution>
            <PASDistributions>25</PASDistributions>


Please help in this regards