def _find_param_separator(tokens):
    """
    Return the index of the param separator.

    :param list tokens: list of tokens on the parameter line
    :returns: integer index of the separator or :data:`None` if no
        separator is found
    :rtype: int

    Different versions of sphinxcontrib-httpdomain/autotornado use different
    renderings for HTTP parameters.  For example ``name (type) -- text``
    where the ``--`` might be a single hyphen or the unicode em-dash...

    """
    idx = [i for i, v in enumerate(tokens) if v in ('\u2013', '--', '-')]
    return idx[0] if idx else None