Sequence of elements of same name but various types

Hi there, 

I am trying to write an XSD file for an existing XML-based file format. The format represents a list of elements, each element being either in the following format: 

<item flavor="text">
	some text
</item>

or in the following format: 

<item flavor="elements">
	<elem />
</item>

In other words, all elements are named "item", but their allowed content depends on the value of the attribute "flavor". If flavor="text", the content must be text that satisfies some regular expression, and if flavor="elements", the content must be an element named "elem". 

Is there a way to write an XSD file for this format? Or is it just not possible because of the Unique Particle Attribution constraint? 

Thanks in advance for your help