protected FactFieldSelector createFactFieldSelector() {
   FactFieldSelector factFieldSelector = new FactFieldSelector();
   for (String fieldName : dmo.getFieldCompletions(fact.getType())) {
     if (!fact.isFieldNameInUse(fieldName)) {
       factFieldSelector.addField(fieldName);
     }
   }
   return factFieldSelector;
 }
 @Override
 public void onSelection(SelectionEvent<String> stringSelectionEvent) {
   fact.getFieldData().add(new FieldPlaceHolder(stringSelectionEvent.getSelectedItem()));
   parent.renderEditor();
 }