DataFactory methods to create from existing term and quad | closes #137

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@@ -337,6 +337,8 @@ <h3><dfn>DataFactory</dfn> interface</h3>
       Variable variable(string value);
       DefaultGraph defaultGraph();
       Quad quad(Term subject, Term predicate, Term object, optional Term? graph);
+      Term fromTerm(Term original);
+      Quad fromQuad(Quad original);
     };
     </pre>
 
@@ -369,6 +371,14 @@ <h3><dfn>DataFactory</dfn> interface</h3>
       If <code>graph</code> is <code>undefined</code> or <code>null</code>
       it MUST set <code>graph</code> to a <code>DefaultGraph</code>.
     </p>
+    <p>
+      <dfn>fromTerm()</dfn> returns a new instance of specific <code>Term</code> subclass,
+      which passed to <code>original.equals()</code> returns <code>true</code>
+    </p>
+    <p>
+      <dfn>fromQuad()</dfn> returns a new instance of <code>Quad</code>,
+      which passed to <code>original.equals()</code> returns <code>true</code>
+    </p>
   </section>
 </section>