XML Schema - How to define any element of type Foe

Hello,

I have a complexType definition, Foe.

In another complexType, I want to allow any child element, but require 
that each child element is valid against Foe definition.

Something like :

<xs:complexType name="Foe">
   <xs:attribute name="start" type="xs:positiveInteger" use="required"/>
   <xs:attribute name="end" type="xs:positiveInteger" use="required"/>
   <xs:assert test="@end ge @start">
</xs:complexType>

<xs:complexType name="parent">
   <xs:sequence>
     <xs:any type="Foe"/>
   </xs:sequence>
</xs:complexType>

How can I do this ?

Thanks,
Christophe