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

def extend_supplement_links(destination, source):
    """
    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
    """
    for key, value in iteritems(source):
        if key not in destination:
            destination[key] = value
        else:
            destination[key].extend(value)