Private wrapper function. Appends ``string`` to the
    docstring of the wrapped function.

def _add_to_docstring(string):
    '''Private wrapper function. Appends ``string`` to the
    docstring of the wrapped function.

    '''
    def wrapper(method):
        if method.__doc__ is not None:
            method.__doc__ += string
        else:
            method.__doc__ = string
        return method
    return wrapper