public string Visit(FeatureSetTopic featureSet, Topic nextTopic, Topic previousTopic)
 {
     if(featureSet.CustomTopic != null) {
         FeatureTemplate featureTemplate = new FeatureTemplate(featureSet.CustomTopic, nextTopic, previousTopic);
         return featureTemplate.TransformText(); 
     }
     FeatureSetTemplate featureSetTemplate = new FeatureSetTemplate(featureSet, nextTopic, previousTopic);
     return featureSetTemplate.TransformText();
 }
 public string Visit(FeatureTopic feature, Topic nextTopic, Topic previousTopic)
 {
     FeatureTemplate page = new FeatureTemplate(feature, nextTopic, previousTopic);
     return page.TransformText();
 }