Return the single instance of wrapped.
    :param wrapped: the wrapped class.
    :param _: unused
    :param args: optional arguments for wrapped object.
    :param kwargs: optional arguments for wrapped object.

def singleton(wrapped, _, args, kwargs):
    """Return the single instance of wrapped.
    :param wrapped: the wrapped class.
    :param _: unused
    :param args: optional arguments for wrapped object.
    :param kwargs: optional arguments for wrapped object.
    """
    if wrapped not in _INSTANCES:
        _INSTANCES[wrapped] = wrapped(*args, **kwargs)
    return _INSTANCES[wrapped]