<xsd:schema
   targetNamespace="http://www.opengis.net/ogc"
   xmlns:ogc="http://www.opengis.net/ogc"
   xmlns:gml="http://www.opengis.net/gml"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema"
   elementFormDefault="qualified">

  <xsd:element name="Add"
               type="ogc:BinaryOperatorType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="Sub"
               type="ogc:BinaryOperatorType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="Mul"
               type="ogc:BinaryOperatorType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="Div"
               type="ogc:BinaryOperatorType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="PropertyName"
               type="ogc:PropertyNameType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="Function"
               type="ogc:FunctionType"
               substitutionGroup="ogc:expression"/>
  <xsd:element name="Literal"
               type="ogc:LiteralType"
               substitutionGroup="ogc:expression"/>

  <xsd:element name="expression" type="ogc:ExpressionType" abstract="true"/>

  <xsd:complexType name="ExpressionType" abstract="true"/>
  <xsd:complexType name="BinaryOperatorType">
    <xsd:complexContent>
      <xsd:extension base="ogc:ExpressionType">
        <xsd:sequence>
          <xsd:element ref="ogc:expression" minOccurs="2" maxOccurs="2"/>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>
  <xsd:complexType name="FunctionType">
    <xsd:complexContent>
      <xsd:extension base="ogc:ExpressionType">
        <xsd:sequence>
          <xsd:element ref="ogc:expression"
                       minOccurs="0" maxOccurs="unbounded"/>
        </xsd:sequence>
        <xsd:attribute name="name" type="xsd:string" use="required"/>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>
  <xsd:complexType name="LiteralType">
    <xsd:complexContent mixed="true">
      <xsd:extension base="ogc:ExpressionType">
        <xsd:sequence>
          <xsd:any minOccurs="0"/>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>
  <xsd:complexType name="PropertyNameType">
    <xsd:complexContent mixed="true">
      <xsd:extension base="ogc:ExpressionType"/>
    </xsd:complexContent>
  </xsd:complexType>
</xsd:schema>

• BinaryOperatorType
• ExpressionType
• FunctionType
• LiteralType
• PropertyNameType

• Add
• Div
• Function
• Literal
• Mul
• PropertyName
• Sub
• expression