code: def extend_supplement_links(destination, source):
      
      for key, value in iteritems(source):
          if key not in destination:
              destination[key] = value
          else:
              destination[key].extend(value)
docs: Extends (merges) destination dictionary with supplement_links
      from source dictionary. Values are expected to be lists, or any
      data structure that has `extend` method.
  
      @param destination: Destination dictionary that will be extended.
      @type destination: @see CourseraOnDemand._extract_links_from_text
  
      @param source: Source dictionary that will be used to extend
          destination dictionary.
      @type source: @see CourseraOnDemand._extract_links_from_text