Help with Groups

Kind Readers,

I'm not quite sure how to express the following in XML Schema:

I want a group such that each item in the group may appear zero or one times
and there must be at least one element present.  For example, element <x>
can contain elements chosen from the set {<a>, <b>, <c>, <d>}.  Each element
in the set may appear at most once and element <x> must not be empty. 

How do I express this in XML Schema?  Any help would be appreciated.

Regards,

David I. Bell
Hewlett-Packard
mailto:David_I_Bell@hp.com
Tel: 650.236-5007