assert: Add _assert_/@class@

This predicates the assertion for each class match.

* assert.xml (_assert_)[@class@]: Add parameter.

core/assert.xml

@@ -42,6 +42,8 @@
         yields~$\top$,
           only one assertion will ever match,
             even if others would match if execution were to continue.}
+  \tt{@class@} may optionally be used to predicate the asseriton itself---%
+    the assertion will be performed only on respective class matches.
 
   \ref{_assert_} implements assertions by genearting two classifications---%
     one to perform the actual assertion,
@@ -60,6 +62,9 @@
       <text>Assertion</text>
     </param>
 
+    <param name="@class@" desc="Only perform assertion for respective class
+                                matches (optional)" />
+
 
     <param name="@neg_as@"
            desc="Generated name for classification to be negated">
@@ -74,7 +79,17 @@
     <!-- The actual assertion will be performed by one classification... -->
     <classify as="@neg_as@" yields="@neg_yields@"
               desc="{@failure@} (assertion result)">
-      <param-copy name="@values@" />
+      <any>
+        <!-- if class is provided and does not match, then the assertion
+             succeeds automatically -->
+        <if name="@class@">
+          <t:match-class name="@class@" value="FALSE" />
+        </if>
+
+        <all>
+          <param-copy name="@values@" />
+        </all>
+      </any>
     </classify>
 
     <!-- ...which is in turn negated for the terminating