def join_list(aList, indent='', leading='', sep='', trailing=''):
    """
    Create a dict representing the concatenation of the
    strings in aList, formatted per the keyword args.
    See the HTMLReportTraverser class for many examples.
    """
    if not aList:
        return None
    assert isinstance(indent, str), indent
    assert isinstance(leading, str), leading
    assert isinstance(sep, str), sep
    assert isinstance(trailing, str), trailing
    if indent or leading or sep or trailing:
        return {'_join_list': True, 'aList': aList, 'indent': indent,
            'leading': leading, 'sep': sep, 'trailing': trailing}
    return aList
