XML tests

I have a schema which validates my XML files very well, but I've recently
added a new optional element to a previous defined element, and I would
like to know if there is a way to test that if an element is present then
only certain other elements can be present.

i.e

old element, can have elements 1,2 and 3

<toplevel>
 <element1>...</element1>
 <element2>...</element2>
 <element3>...</element3>
</toplevel>

new element, if new element is present we can only have element 1.

<toplevel>
 <newelement/>
 <element1>...</element1>
</toplevel>

Cheers
Stuart