( A or B ) or both

Hello,

For an XML Schema complexType, I would like to specify such that it must 
contain either of the elements A or B, but it /may/ also contain them 
both(but at a minimum must A or B be there). In other words, I'm talking 
about a logical OR. 

How is this done? If i put minOccurs="0" on A and B, that allows none of them 
to be there. Somehow must the relation between the two be expressed.

BTW, for another Schema of mine, I need to allow XHTML markup in some 
elements, in a way possible to validate. Should I simply reference the 
"block" element group in XHTML 1.0's XSD?


Cheers,

		Frans