Allow graphs in dataset to be identified by blank nodes

spec/index.html

@@ -7879,7 +7879,7 @@ <h4>RDF Dataset</h4>
             <p>For the following definitions, we capture each RDF dataset as a set:<br>
               { G, (&lt;u<sub>1</sub>&gt;, G<sub>1</sub>), (&lt;u<sub>2</sub>&gt;, G<sub>2</sub>), . .
               . (&lt;u<sub>n</sub>&gt;, G<sub>n</sub>) }<br>
-              where G and each G<sub>i</sub> are graphs, and each &lt;u<sub>i</sub>&gt; is an IRI. Each
+              where G and each G<sub>i</sub> are graphs, and each &lt;u<sub>i</sub>&gt; is an IRI or blank node. Each
               &lt;u<sub>i</sub>&gt; is distinct.</p>
             <p>G is called the default graph. (&lt;u<sub>i</sub>&gt;, G<sub>i</sub>) are called named
               graphs.</p>