How to apply 'order by' in RetrieveMultiple operation using soap ?

<RetrieveMultiple xmlns="schemas.microsoft.com/.../Services" xmlns:i="www.w3.org/.../XMLSchema-instance">
<query i:type="a:QueryExpression" xmlns:a="schemas.microsoft.com/.../Contracts">
<a:ColumnSet>
<a:AllColumns>true</a:AllColumns>
<a:Columns xmlns:b="schemas.microsoft.com/.../Arrays" />
</a:ColumnSet>
<a:Criteria>
</a:Criteria>
<a:Distinct>false</a:Distinct>
<a:EntityName>contact</a:EntityName>
<a:LinkEntities />
<a:orders>
<a:order>
<a:attributename>firstname</a:attributename>
<a:ordertype>0</a:ordertype>
</a:order>
</a:orders>
<a:NoLock>false</a:NoLock>
</query>
</RetrieveMultiple>