we use more_itertools flatten to do a much better job

flatten.py

from typing import Dict, Set
from more_itertools import flatten


def flatten_multivalues(key_to_values: Dict[str, Set[int]]) -> Set[int]:
    return set(flatten(key_to_values.values()))


def test_flattens_multivalue_dicts():
    # shamelessly taken from: https://www.elastic.co/blog/found-elasticsearch-from-the-bottom-up
    term_to_document_indices = {
        "choice": {3},
        "coming": {1},
        "fury": {2},
        "is": {1, 2, 3},
        "the": {2, 3},
    }

    all_document_indices = flatten_multivalues(term_to_document_indices)

    assert all_document_indices == {1, 2, 3}