dotnet xml: How can one use a choice and a sequence group in the same schema?

I have 2 child elements that can appear once each but in any order, and 2
child elements that can appear any number of times.

eg

<example>
<child1>Any order, appear once</child1>
<chid2>Any order, appear once</child2>
<chid3>Any order, appear any number of times</child3>
<chid3>Any order, appear any number of times</child3>
<child4>Any order, appear any number of times</child4>
<child4>Any order, appear any number of times</child4>
</example>