Need help with any and all

I'm trying to develop a schema that requires element X to:

    have exactly one each of child elements A, B and C in any order
    allow other child elements from any other namespace

I've been able to develop schemas that meet two of my three requirements but
not all three.  I've also been able to develop a schema that appears to meet
all three, however validation succeeds when any of the required elements are
omitted.  If anyone else has done something similar, I would really
appreciate any guidance you have to offer.

Brenda Bell
Sr. Software Architect
Juice Software, Inc.
Phone: 603.428.3994
Cell: 603.494.8206
Fax: 603.428.8713
Email: bbell@juicesoftware.com