order of attributes

Hi all,
Using a xml schema, how is it possible to impose the order of the attributes on a element in a xml instance?
eg : on the element 'elem1' I want the attributes to appear in the order : first 'att1' then 'att2' and finally 'att3'.

<elem1 att1="blabla" att2="blabla" att3="blabla" /> valid

<elem1 att3="blabla" att2="blabla" att1="blabla" /> NOT valid

How can specify such a constraint ?
Thank you for your anwers.
JF