List raw data IDs grouped by symbol ID from a pickle file
       ``path_to_data``.

def _list_ids(path_to_data):
    """List raw data IDs grouped by symbol ID from a pickle file
       ``path_to_data``."""
    loaded = pickle.load(open(path_to_data, "rb"))
    raw_datasets = loaded['handwriting_datasets']
    raw_ids = {}
    for raw_dataset in raw_datasets:
        raw_data_id = raw_dataset['handwriting'].raw_data_id
        if raw_dataset['formula_id'] not in raw_ids:
            raw_ids[raw_dataset['formula_id']] = [raw_data_id]
        else:
            raw_ids[raw_dataset['formula_id']].append(raw_data_id)
    for symbol_id in sorted(raw_ids):
        print("%i: %s" % (symbol_id, sorted(raw_ids[symbol_id])))