--- a/model/prov-n.html	Tue May 15 10:53:42 2012 +0100
+++ b/model/prov-n.html	Tue May 15 11:53:54 2012 +0100
@@ -567,7 +567,7 @@
 <section id="prov-n-standard-terms"> 
 <h3>Identifiers and attributes</h3>
 
-<p>Most expressions defined in the grammar include the use of two terms: an identifier for the predicate, and a set of attribute-value pairs, delimited by square brackets. Both are optional (unless specified otherwise). By convention, the identifier is the first term in any expression, followed by a semi-colon ';' and the  set of attribute-value pairs is the last. </p>
+<p>Most expressions defined in the grammar include the use of two terms: an identifier for the predicate, and a set of attribute-value pairs, delimited by square brackets. Identifiers are optional except for Entities, Activites, and Agents. Identifiers are always the first term in any expression. By convention, </em>optional</em> identifiers are separated using a semi-colon ';'. This makes it possible to completely omit an optional identifier with no ambiguity arising. Also, the  set of attribute-value pairs is always the last term in any expression. </p>
 
 <div class="anexample">
 <p>Derivation has an optional identifier. In the first expression, the identifier is not available, while it is explicit in the second. The third example shows that one can optionally indicate the missing identifier using the <span class="name">-</span> marker.</p>
@@ -578,9 +578,14 @@
 </pre>
 </div>
 
-A distinction is made between expressions with no attributes, and expressions that include an empty list of attributes.
+Lack of attributes can be equivalently expressed by omitting the list, or by using an empty list
+
+<!-- Expressions with no attributes A distinction is made between expressions with no attributes, and expressions that include an empty list of attributes.  -->
+
 <div class="anexample">
-<p>The first activity does not have any attributes. The second has an empty list of attributes. The third activity  has two attributes. 
+<p>The first and second activities have no attributes.
+<!-- does not have any attributes. The second has an empty list of attributes. -->
+The third activity  has two attributes. 
 <pre class="codeexample" >
 activity(ex:a10)
 activity(ex:a10, [])