add value accessor for mapping

Helpers.jrag

@@ -14,4 +14,13 @@ aspect Helpers {
   public void MappingElement.addKeyValuePair(String key, Element value) {
     addKeyValuePair(new KeyValuePair(key, value));
   }
+
+  public java.util.Optional<Element> MappingElement.getValue(String key) {
+    for (KeyValuePair pair : getKeyValuePairList()) {
+      if (pair.getKey().equals(key)) {
+        return java.util.Optional.of(pair.getValue());
+      }
+    }
+    return java.util.Optional.empty();
+  }
 }