<element name=“addressBook” xmlns=“relaxng.org/ns/structure/1.0”>

<zeroOrMore>
  <element name="card">
    <element name="name">
      <text/>
    </element>
    <element name="email">
      <text/>
    </element>
    <optional>
      <element name="prefersHTML">
        <empty/>
      </element>
    </optional>
  </element>
</zeroOrMore>

</element>