def _norm_args(norm):
    """
    Returns the proper normalization parameter values.

    Possible `norm` values are "backward" (alias of None), "ortho",
    "forward".

    This function is used by both the builders and the interfaces.

    """
    if norm == "ortho":
        ortho = True
        normalise_idft = False
    elif norm is None or norm == "backward":
        ortho = False
        normalise_idft = True
    elif norm == "forward":
        ortho = False
        normalise_idft = False
    else:
        raise ValueError(f'Invalid norm value {norm}; should be "ortho", '
                         '"backward" or "forward".')
    return dict(normalise_idft=normalise_idft, ortho=ortho)