def add_script(req, filename, mimetype='text/javascript'):
    """Add a reference to an external javascript file to the template.

    If the filename is absolute (i.e. starts with a slash), the generated link
    will be based off the application root path. If it is relative, the link
    will be based off the `/chrome/` path.
    """
    scriptset = req.chrome.setdefault('scriptset', set())
    if filename in scriptset:
        return False # Already added that script

    if filename.startswith('common/') and 'htdocs_location' in req.chrome:
        href = Href(req.chrome['htdocs_location'])
        path = filename[7:]
    else:
        href = req.href
        if not filename.startswith('/'):
            href = href.chrome
        path = filename
    script = {'href': href(path), 'type': mimetype}

    req.chrome.setdefault('scripts', []).append(script)
    scriptset.add(filename)