Is there a way in XML schema to validate elements A, B, AB, BA, but not
permit duplicates like AA, BB, ABA, etc, and here's the hard part, not
validate unless at least one of A or B exists?

It's like saying, "I want one or the other or both, but not more than
one of either, and order doesn't matter to me."

This is how far I got after much testing, but it unfortunately
validates when there are *neither* A nor B elements present:

    <xs:complexType name="Name">
        <xs:all>
            <xs:element name="A" type="xs:string" minOccurs="0"/>
            <xs:element name="B" type="xs:string" minOccurs="0"/>
        </xs:all>
    </xs:complexType>

Here's my test cases that should validate (each line is a separate
document):
good:    <A/>
good:    <B/>
good:    <A/><B/>
good:    <B/><A/>
Here's my test cases where it should *not* validate:
good:    <A/><A/>
good:    <B/><B/>
bad:     <!-- empty -->

Everything works except the very last test case.  I want it to fail
validation when nothing is present, but it still validates just fine.
Ugh.  Any advice?