def _merge_dicts_of_sets(current, new):
    """Merge the elements of `new` into `current` (via set union).

    Both `new` and `current` must be `dict`s whose values are `set`s.
    This modifies `current`.

    """
    for key in new:
        if key in current:
            current[key] |= new[key]
        else:
            current[key] = new[key]
