--- a/model/prov-constraints.html	Thu Sep 06 08:55:35 2012 +0200
+++ b/model/prov-constraints.html	Thu Sep 06 08:34:50 2012 +0100
@@ -422,9 +422,8 @@
 
 
 <p> This document defines a subset of PROV instances called
-<i><a>valid</a></i> PROV instances.  A <a>valid</a> PROV instance corresponds to a
-consistent history of objects and interactions to which logical
-reasoning can be safely applied.
+<i><a>valid</a></i> PROV instances.  
+The intent of validation is ensure that a PROV instance represents a history of objects and their interactions which is consistent, and thus safe to use for the purpose of logical reasoning and other kinds of analysis.
 Valid PROV instances satisfy
 certain <a>definitions</a>, <a>inferences</a>, and
 <a>constraints</a>. These definitions, inferences, and constraints
@@ -1416,9 +1415,9 @@
 </section>
 
 <section>
-<h4>Summary of definitions, inferences, and constraints</h4>
-
-<p><a href="">Table 2</a> summarizes the definitions, inferences, and
+<h4>Summary of inferences and constraints</h4>
+
+<p><a href="">Table 2</a> summarizes the inferences, and
 constraints specified in this document, broken down by component and
 type or relation involved.
 </p>
@@ -1427,7 +1426,7 @@
 
 <div id="prov-constraints-fig" style="text-align: left;">
 <table  class="thinborder" style="margin-left: auto; margin-right: auto; border-color: #0;">
-<caption id="prov-constraints">Table 2: Summary of definitions, constraints, and inferences for PROV Types and Relations</caption>
+<caption id="prov-constraints">Table 2: Summary of inferences and constraints for PROV Types and Relations</caption>
 <tr><td><a><b>Type or Relation Name</b></a></td><td><b>
   Inferences and Constraints</b></td><td><b>Component</b></td></tr>
 <tr><td style="border-width: 0px; "></td><td style="border-width: 0px; "></td><td style="border-width: 0px; "></td></tr>
@@ -1989,7 +1988,8 @@
 <!--<div id="optional-attributes1"> -->
 <p id="optional-placeholders_text">Finally,  most optional parameters (written <span
   class="name">-</span>) are, for the purpose of this document,
-  considered to be distinct, fresh existential variables.  Thus,
+  considered to be distinct, fresh existential variables.  Optional parameters are defined in  [[!PROV-DM]] and in [[!PROV-N]] for each type of PROV statement.
+  Thus,
   before proceeding to apply other definitions or inferences, most
   occurrences of <span class="name">-</span> are to be replaced
  by fresh existential variables, distinct from any others occurring in
@@ -2102,11 +2102,17 @@
 <p> <a class="rule-ref"
   href="#optional-placeholders"><span>TBD</span></a> states how parameters are to be expanded,
     using the expandable parameters defined in   <a
-  href="#expandable-parameters-fig">Table 3</a>.  The last two parts
+  href="#expandable-parameters-fig">Table 3</a>.  The last two parts, 4 and 5,
   indicate how to handle expansion of parameters for
   <span class="name">wasDerivedFrom</span> expansion, which is only allowed for the
-  generation and use parameters when the activity is specified.  A
-  later constraint, <a class="rule-ref"
+  generation and use parameters when the activity is specified.
+  Essentially, the definitions state that parameters  
+<span class="name">g,u</span> are expandable only if the activity is specified, i.e., if parameter <span class="name">a</span> is provided.
+
+The rationale for this is that when a is provided, then there have to be two events, namely <span class="name">u</span> and <span class="name">g</span>, which account for the usage of <span class="name">e1</span> and the generation of <span class="name">e2</span>, respectively, by <span class="name">a</span>. Conversely, if <span class="name">a</span> is not provided, then one cannot tell whether one or more activities are involved in the derivation, and the explicit introduction of such events, which correspond to a single acitivity, would therefore not be justified.  </p>
+
+
+<p>  A  later constraint, <a class="rule-ref"
   href="#impossible-unspecified-derivation-generation-use"><span>TBD</span></a>,
   forbids specifying generation and use parameters when the activity
   is unspecified.</p>
@@ -3216,7 +3222,7 @@
 implies event ordering, or vice versa.  However, an application MAY flag time values
 that appear inconsistent with the event ordering as possible
 inconsistencies.  When generating provenance, an application SHOULD
-use a consistent timeline for related PROV statements within an
+use a consistent imeline for related PROV statements within an
 instance.</p>