<?xml version=“1.0”?> <xsd:schema xmlns:xsd=“www.w3.org/2001/XMLSchema”>

<xsd:simpleType name="myDecimal">
        <xsd:restriction base="xsd:decimal">                    
                <xsd:totalDigits value="18"/>                   
        </xsd:restriction>
</xsd:simpleType>

<xsd:element name="foo">
        <xsd:complexType>
                <xsd:sequence>
                        <xsd:element name="val" type="myDecimal" maxOccurs="unbounded"/>
                        <xsd:element name="ref" type="myDecimal" minOccurs="0"/>
                </xsd:sequence>
        </xsd:complexType>

        <!--xsd:key name="myKey">
                <xsd:selector xpath="val"/>
                <xsd:field xpath="."/>
        </xsd:key>

        <xsd:keyref name="myKeyRef" refer="myKey">
                <xsd:selector xpath="ref"/>
                <xsd:field xpath="."/>
        </xsd:keyref-->
</xsd:element>

</xsd:schema>