XML Schema: element A, element B or both

Question

I'm trying to write an XML schema construct that would allow to have either element A or element B, or both A and B.

I've tried this:

<xs:complexType name="RoleMapType">
    <xs:sequence>
        <xs:element minOccurs="0" name="Description" type="xs:string"/>
        <xs:element minOccurs="0" name="Schedule" type="xs:string"/>
        <xs:choice>
            <xs:element name="Loader" type="RoleMapLoaderType"/>
            <xs:element name="Dump" type="RoleMapDumpType"/>
            <xs:sequence>
                <xs:element name="Loader" type="RoleMapLoaderType"/>
                <xs:element name="Dump" type="RoleMapDumpType"/>
            </xs:sequence>
        </xs:choice>
    </xs:sequence>
</xs:complexType>

but I'm getting an error saying that Loader and Loader would be ambiguous.

How can I enforce such a constraint?

Answer 1

I think the following is not considered ambiguous:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  
<xs:complexType name="RoleMapType">
    <xs:sequence>
        <xs:element minOccurs="0" name="Description" type="xs:string"/>
        <xs:element minOccurs="0" name="Schedule" type="xs:string"/>
        <xs:choice>
          <xs:sequence>
            <xs:element name="Loader" type="RoleMapLoaderType"/>
            <xs:element name="Dump" type="RoleMapDumpType" minOccurs="0"/>            
          </xs:sequence>
          <xs:element name="Dump" type="RoleMapDumpType"/>
        </xs:choice>
    </xs:sequence>
</xs:complexType>

<xs:complexType name="RoleMapLoaderType"/>

<xs:complexType name="RoleMapDumpType"/>

</xs:schema>