def absolute_url (base_url, base_ref, parent_url):
    """
    Search for the absolute url to detect the link type. This does not
    join any url fragments together!

    @param base_url: base url from a link tag
    @type base_url: string or None
    @param base_ref: base url from <base> tag
    @type base_ref: string or None
    @param parent_url: url of parent document
    @type parent_url: string or None
    """
    if base_url and urlutil.url_is_absolute(base_url):
        return base_url
    elif base_ref and urlutil.url_is_absolute(base_ref):
        return base_ref
    elif parent_url and urlutil.url_is_absolute(parent_url):
        return parent_url
    return u""
Search for the absolute url to detect the link type. This does not
    join any url fragments together!

    @param base_url: base url from a link tag
    @type base_url: string or None
    @param base_ref: base url from <base> tag
    @type base_ref: string or None
    @param parent_url: url of parent document
    @type parent_url: string or None