def add_meta(req, content, http_equiv=None, name=None, scheme=None, lang=None):
    """Add a `<meta>` tag into the `<head>` of the generated HTML."""
    meta = {'content': content, 'http-equiv': http_equiv, 'name': name,
        'scheme': scheme, 'lang': lang, 'xml:lang': lang}
    req.chrome.setdefault('metas', []).append(meta)
