Question about checking dependencies between fields in xml document via xsd

Hello.

 

During a few days I can't solve one problem.

There are two groups of fields in the xml file

first

<field1></field1>
<field2></field2>
<field3></field3>

second

<field4></field4>
<field5></field5>

 

All fields in the same level of document.

I need to check filled/presents or not at least one of these groups.

 

How to do it in xsd?

 

 

Thanks.

 

With best regards

Dmitry Ionov