--- a/model/prov-dm-constraints.html	Mon Apr 02 15:56:29 2012 +0100
+++ b/model/prov-dm-constraints.html	Mon Apr 02 16:37:33 2012 +0100
@@ -628,6 +628,7 @@
 
 
 
+
 <p>
 A generation's id is OPTIONAL. It MUST be used when annotating generations or when defining
 derivations (see <a href="#Derivation-Relation">Derivation</a>).
@@ -639,10 +640,18 @@
 </div>
 
 
+<p>A generation can be used to indicate a generation time without having to specify the involved activity.  A generation time is unique, as specified by the following constraint.<p> 
+
+<div class='constraint' id='unique-generation-time'>
+Given an entity denoted by <span class="name">e</span> and 
+two time instants  <span class="name">t1</span> and <span
+class="name">t2</span>,
+<span class='conditional'>if</span> <span class="name">wasGeneratedBy(e, -, t1)</span> and <span class="name">wasGeneratedBy(e, -, t2)</span> hold, then <span class="name">t1</span>=<span class="name">t2</span>.
+</div> 
 
 <p></p>
 <div class="structural-forward">
-See <a href="#generation-uniqueness">generation-uniqueness</a> for a structural constraint on generations.
+See <a href="#generation-uniqueness">generation-uniqueness</a> for further structural constraints on generations.
 </div>
 
 
@@ -1530,10 +1539,11 @@
 </p>
 
 <div class='constraint' id='generation-uniqueness'>Given an entity denoted by <span class="name">e</span>, two activities denoted by <span class="name">a1</span> and <span
-class="name">a2</span>, and two sets of attribute-value pairs <span class="name">attrs1</span> and <span class="name">attrs2</span>,
-<span class='conditional'>if</span> <span class="name">wasGeneratedBy(id1,e,a1,attrs1)</span> and <span class="name">wasGeneratedBy(id2,e,a2,attrs2)</span> exist in the scope of a given
+class="name">a2</span>, two time instants  <span class="name">t1</span> and <span
+class="name">t2</span>, and two sets of attribute-value pairs <span class="name">attrs1</span> and <span class="name">attrs2</span>,
+<span class='conditional'>if</span> <span class="name">wasGeneratedBy(id1, e, a1, t1, attrs1)</span> and <span class="name">wasGeneratedBy(id2, e, a2, t2, attrs2)</span> exist in the scope of a given
 account,
-<span class='conditional'>then</span> <span class="name">id1</span>=<span class="name">id2</span>, <span class="name">a1</span>=<span class="name">a2</span>  and <span class="name">attrs1</span>=<span class="name">attrs2</span>.
+<span class='conditional'>then</span> <span class="name">id1</span>=<span class="name">id2</span>, <span class="name">a1</span>=<span class="name">a2</span>, <span class="name">t1</span>=<span class="name">t2</span>  and <span class="name">attrs1</span>=<span class="name">attrs2</span>.
 </div>