I want to match these two documents:
<a>
<b/>
</a>
and:
<a>
<b/>
<c/>
<d/>
</a>
That is, there must be either neither of c and d or both of c and d.
I've tried to have a choice of two sequences, but I get: "The content model is
not determinist.", I guess because the choice has to be disambiguated on the
first element, which is the same for the two different documents.
You can put c and d inside a sequence group with minOccurs=0. Like this:
<element name="a">
<complexType>
<sequence>
<element name="b" />
<sequence minOccurs="0">
<element name="c" />
<element name="d" />
</sequence>
</sequence>
</complexType>
</element>
If you want c and d to be able to appear in any order, you can do this:
<element name="a">
<complexType>
<sequence>
<element name="b" />
<choice minOccurs="0">
<sequence>
<element name="c" />
<element name="d" />
</sequence>
<sequence>
<element name="d" />
<element name="c" />
</sequence>
</choice>
</sequence>
</complexType>
</element>
Obviously, this approach does not scale well if you have more than two members in the group.