Optional list of node labels in spec

docs/index.html

@@ -138,7 +138,7 @@ <h2>Metadata</h2>
         </section>
         <section id='node'>
             <h2>Node</h2>
-            <p>A <dfn>node declaration</dfn> begins with the node identifier, followed by a list of node labels and
+            <p>A <dfn>node declaration</dfn> begins with the node identifier, followed by an optional list of node labels and
                 optional node properties, optional list of graph names and optional annotation list.</p>
             <aside class="example" title="Node declaration">
                 <p>The example presents node declaration for node <code>EI01</code>. This node have two labels
@@ -213,7 +213,7 @@ <h2>YARS-PG Schema Language</h2>
         <p>An <dfn>YARS-PG schemas</dfn> may be used to determine the expected structure of a YARS-PG document.</p>
         <section id='node-schema'>
             <h2>Node schema</h2>
-            <p>A <dfn>node schema declaration</dfn> begins with list of node labels, followed by optional properties of
+            <p>A <dfn>node schema declaration</dfn> begins with optional list of node labels, followed by optional properties of
                 the node with its value type, optional list of graph names and optional annotation list.</p>
             <aside class="example" title="Node schema declaration for corresponding node">
                 <p>The example presents node schema declaration with the corresponding node declaration. Node schema