<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <!-- Define the XML Schema of a transaction -->
  <xs:element name="instance">
    <xs:complexType>
      <xs:sequence>

        <xs:element name="parameter" minOccurs="0" maxOccurs="unbounded">
          <xs:complexType mixed="true">
            <xs:attribute name="id" use="required" type="xs:string"/>
          </xs:complexType>
        </xs:element>

      </xs:sequence>
    </xs:complexType>

    <xs:unique name="unique_instance_parameter_id">
      <xs:selector xpath="./parameter"/>
      <xs:field xpath="@id"/>
    </xs:unique>

  </xs:element>
</xs:schema>
