title: Deleting triples identified by subject and predicate only
body: <p>I can delete a triple identifying it by each of the three elements:</p>
  
  <pre><code>DELETE DATA 
  { GRAPH &lt;http://my.graph.com&gt;
     {
        &lt;http://my.domain.com/foo&gt; &lt;http://standard.com/catalog#title&gt; "ABC" .
     }
  };
  </code></pre>
  
  <p>If I omit <code>"ABC"</code>I get an error.  How can I delete triples identifying them by only the first two elements?</p>