Return a dict ``{column_id: [possible term ids]}``
    with collision information.

def _get_collisions(indices):
    # type: (...) -> Dict[int, List[int]]
    """
    Return a dict ``{column_id: [possible term ids]}``
    with collision information.
    """
    collisions = defaultdict(list)  # type: Dict[int, List[int]]
    for term_id, hash_id in enumerate(indices):
        collisions[hash_id].append(term_id)
    return dict(collisions)