<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="namespace_uri" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="a">
    <xs:complexType mixed="true">
      <xs:sequence>
        <xs:element ref="x:x" minOccurs="0" xmlns:x="x_uri"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>