def concat_strings(string_list):
    """
    Concatenate all the strings in possibly-nested string_list.

    @param list[str]|str string_list: string(s) to concatenate
    @rtype: str

    >>> list_ = 'cat'
    >>> concat_strings(list_)
    'cat'
    >>> list_ = ['cat', 'dog']
    >>> concat_strings(list_)
    'catdog'
    >>> list_ = ["how", ["now", "brown"], "cow"]
    >>> concat_strings(list_)
    'hownowbrowncow'
    """
    # if this isn't a list, ie. it's an innermost element, don't recurse
    if not isinstance(string_list, list):
        return string_list
    else:
        return ''.join([concat_strings(x) for x in string_list])