1) Using "if" control expressions in an XSD <assert>
The XPath 2.0 "if" expression is an useful facility for an <assert>. I'll explain this, via a XML schema validation example, as mentioned below.
XML Schema 1.1 document:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="X">
<xs:complexType>
<xs:sequence>
<xs:choice>
<xs:element name="b" type="xs:integer"/>
<xs:element name="c" type="xs:integer"/>
</xs:choice>
<xs:element name="a" type="xs:integer"/>
</xs:sequence>
<xs:attribute name="el" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="b"/>
<xs:enumeration value="c"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:assert test="if (@el = 'b') then b else c"/>
</xs:complexType>
</xs:element>
</xs:schema>
(the XSD <choice> specifies, that either element "b" should occur or the element "c" should occur. this is further controlled by an <assert> constraint, which specifies that if value of attribute "el" is 'b' then element "b" should occur, otherwise element "c" should occur for the <choice>.)
Below are few XML instance documents, that can be validated with the above XSD document:
<X el="b">
<b>100</b>
<a>200</a>
</X>
(valid document)
<X el="b">
<c>100</c>
<a>200</a>
</X>
(invalid document, since the preceding-sibling of element "a" must be element "b")
<X el="p">
<b>100</b>
<a>200</a>
</X>
(invalid document, since the value of attribute "el" is not as per the simpleType definition of attribute "el". the <assert> would also fail.)
2) Using XSD <alternative> instead of <assert>
The XSD example mentioned in point 1) above, can easily be converted to an XSD document using <alternative> to solve the same use case. Below is a modified XSD 1.1 document, using <alternative> element solving the same use case as mentioned in point 1) above.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="X">
<xs:alternative test="@el = 'b'" type="B_type"/>
<xs:alternative test="@el = 'c'" type="C_type"/>
<xs:alternative type="xs:error"/>
</xs:element>
<xs:complexType name="B_type">
<xs:sequence>
<xs:element name="b" type="xs:integer"/>
<xs:element name="a" type="xs:integer"/>
</xs:sequence>
<xs:attribute name="el" type="xs:string" use="required"/>
</xs:complexType>
<xs:complexType name="C_type">
<xs:sequence>
<xs:element name="c" type="xs:integer"/>
<xs:element name="a" type="xs:integer"/>
</xs:sequence>
<xs:attribute name="el" type="xs:string" use="required"/>
</xs:complexType>
</xs:schema>
