code: def absolute_url (base_url, base_ref, parent_url):
      
      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""
docs: 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