def _process_how_args(merge, how, how_subds):
    """Resolve how-related arguments into `how` and `how_subds` values.
    """
    # Translate old --merge value onto --how
    if merge and (how or how_subds):
        raise ValueError("`merge` is incompatible with `how` and `how_subds`")
    elif merge == "ff-only":
        how = merge
    elif merge:
        how = "merge"

    if how == "fetch":
        how = None

    # Map "fetch" to None for easier conditions.
    if how_subds == "fetch":
        how_subds = None
    elif how_subds is None:
        # Subdatasets are updated according to --how unless --how-subds is
        # given.
        how_subds = how
    return how, how_subds