title: Neo4j query to get a document with no parent
body: <p>I have a relation in neo4j</p>
  <pre><code>(d:Document)-[r:HAS_PARENT]-&gt;(p:Document)
  </code></pre>
  <p>but sometime my document have parents.</p>
  <p>I want to fetch documents who are not linked with other document by the HAS_PARENT relation ship</p>
  <p>How can I do ?</p>
  <p>Thanks in advance</p>