def add_link(req, rel, href, title=None, mimetype=None, classname=None, **attrs
    ):
    """Add a link to the chrome info that will be inserted as <link> element in
    the <head> of the generated HTML
    """
    linkid = '%s:%s' % (rel, href)
    linkset = req.chrome.setdefault('linkset', set())
    if linkid in linkset:
        return
    link = {'href': href, 'title': title, 'type': mimetype, 'class': classname}
    link.update(attrs)
    links = req.chrome.setdefault('links', {})
    links.setdefault(rel, []).append(link)
    linkset.add(linkid)
