code: function multiMapSet(multimap, key, value) {
    if (!multimap.has(key)) {
      multimap.set(key, new Set());
    }
    const values = multimap.get(key);
    if (!values.has(value)) {
      values.add(value);
      return true;
    }
    return false;
  }
docs: Given a multimap that uses sets to collect values,
  adds the value to the set for the given key.