- fixes issue #7

sigprops.xsd

@@ -94,7 +94,7 @@
                     <xs:documentation>Name of the significant property.</xs:documentation>
                 </xs:annotation>
             </xs:element>
-            <xs:element name="PropertyExplanation" minOccurs="0" maxOccurs="1">
+            <xs:element name="PropertyExplanation" minOccurs="1" maxOccurs="1">
                 <xs:annotation>
                     <xs:documentation>More detailed explanation of why this property is
                         significant.</xs:documentation>