xsd: attribute points to element id

I have an xml describing vertices on a directed graph:

<graph>
  <vertex id="a">
    <edge target="b">
  </vertex>
  <vertex id="b"/>
</graph>

Given the schema:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://ift.tt/tphNwY">

  <xs:complexType name="vertex">
    <xs:sequence>
      <xs:element type="edge" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="id" use="required" type="xs:string"/>
  </xs:complexType>

  <xs:complexType name="edge">
    <xs:attribute name="target" use="required"/>
  </xs:complexType>

  <xs:complexType name="graph">
    <xs:sequence>
      <xs:element name="vertex" type="vertex" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>
  <xs:element name="crawl-job" type="crawl-jobType"/>
</xs:schema>

How can I modify the target attribute so it's value can only be valid vertex id's?

so my initial example is a valid xml but:

<graph>
  <vertex id="a">
    <edge target="c">
  </vertex>
  <vertex id="b"/>
</graph>

is not.