XML Schema Either/Or or Both

[msalamon]

I want to create a complex element that will have the following type of structure:

<parent>
<child1/>
<child2/>
</parent>

The requiremenets are:
EITHER child1 OR child2 must exist, or
BOTH child1 AND child2 must exist

However, it is NOT acceptable if NEITHER child elements exist. (I would also like to allow 0 to unbounded number of child1 elements.)

An example of this is a traditional C-type &quot;switch&quot; statement, where a user can have 0 to N &quot;case&quot; statements, and/or 0 or 1 &quot;default&quot; statements, but cannot be missing both.

Is there any way, using XML Schema language, to ensure that an XML document conforms to these requirements?