code: public static Stream<String> propertiesReferenced( ICondition condition)
      {
      if( condition == null)
        {
        return Stream.empty();
        }
      
      PropertyVisitor propertyVisitor = new PropertyVisitor();
      condition.accept( propertyVisitor);
      return propertyVisitor.propertiesVisited();
      }
docs: Returns the properties referenced by the given condition.