How can i include condition in XML SCHEMA

Hi People,
I would do this operation:

- I'have an XML Schema and i would insert a condition
                   ....
                  <xs:element name="WEB_PERMISSION" type="xs:string"
ls:isVisible="true" ls:isDisabled="false" ls:caption="TEXT"/>
                 <xs:element name="PERMISSION1" type="MenuTypeInser"
ls:isVisible="true" ls:isDisabled="false" ls:caption="TEXT"/>
                 <xs:element name="PERMISSION2" default="TEXT1"
ls:isVisible="true" ls:isDisabled="false" ls:caption="TEXTa"/>
                   ...

if field WEB_PERMISSION contain the value 1 ----> PERMISSION1;
if field WEB_PERMISSION contain the value 0 ----> PERMISSION2;

Is it possible? How can i do?

Thanks

Dott. Andrea Merli