Implement _rtop_/@negate@

core/numeric/percent.xml

@@ -70,7 +70,6 @@
       <text>false</text>
     </param>
 
-    <!-- TODO -->
     <param name="@negate@" desc="Whether to negate percentage before
                                  conversion">
       <text>false</text>
@@ -78,6 +77,10 @@
 
 
     <c:product>
+      <if name="@negate@" eq="true">
+        <c:const value="-1" desc="Negate result" />
+      </if>
+
       <c:sum>
         <c:value-of name="@name@" index="@index@" />
 

core/test/core/numeric/percent.xml

@@ -138,5 +138,41 @@
         </t:expect>
       </t:it>
     </t:describe>
+
+
+    <t:describe name="given @negative@">
+      \ref{_rtop_} provides a way to negate the result using a
+      short-hand form for convenience and conciseness.
+
+      <t:it desc="negates resulting percentage">
+        <t:given>
+          <t:rtop name="#0.05" negate="true" />
+        </t:given>
+
+        <t:expect>
+          <t:match-result>
+            <c:eq>
+              <c:value-of name="#-5" />
+            </c:eq>
+          </t:match-result>
+        </t:expect>
+      </t:it>
+
+
+      <t:it desc="negates resulting difference">
+        <t:given>
+          <t:rtop name="#2" negate="true"
+                  difference="true" />
+        </t:given>
+
+        <t:expect>
+          <t:match-result>
+            <c:eq>
+              <c:value-of name="#-100" />
+            </c:eq>
+          </t:match-result>
+        </t:expect>
+      </t:it>
+    </t:describe>
   </t:describe>
 </package>