--- a/model/optional.html	Fri Mar 30 16:03:20 2012 +0200
+++ b/model/optional.html	Fri Mar 30 15:51:59 2012 +0100
@@ -3,6 +3,8 @@
 <h3>Optional Attributes</h3>
 
 <h4>Time related attributes</h4>
+
+
 <ol>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#activity.startTime">startTime</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#activity.endTime">endTime</a>
@@ -14,12 +16,12 @@
 
 <h4>Optional entity/agent/activity</h4>
 <ol>
-<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#generation.activity">activity</a>
-<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#start.trigger">trigger</a>
-<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#end.trigger">trigger</a>
-<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#association.agent">agent</a>
+<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#generation.activity">activity</a> (existence of activity implied, if not specified)
+<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#start.trigger">trigger</a> (existence of trigger implied, if not specified)
+<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#end.trigger">trigger</a> (existence of trigger implied, if not specified)
+<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#association.agent">agent</a> (existence of agent implied, if not specified)
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#association.plan">plan</a>
-<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#responsibility.activity">activity</a>
+<li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#responsibility.activity">activity</a> (existence of activity implied, if not specified)
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#revision.responsibility">responsibility</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#quotation.quoterAgent">quoterAgent</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#quotation.originalAgent">originalAgent</a>
@@ -35,6 +37,8 @@
 
 
 <h4>Optional attributes</h4>
+
+Existence of list of attributes (empty or not) is implied, if not specified.
 <ol>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#entity.attributes">attributes</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#activity.attributes">attributes</a>
@@ -59,6 +63,8 @@
 
 
 <h4>Optional id</h4>
+
+Existence of identifier implied, if not specified.
 <ol>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#generation.id">id</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#usage.id">id</a>