code: function get(cache, keys) {
    if (keys.length === 0) {
      return cache.get(sentinel);
    }
    const [key, ...restKeys] = keys;
    const nestedCache = cache.get(key);
    if (!nestedCache) {
      return undefined;
    }
    return get(nestedCache, restKeys);
  }
docs: Get a value from the given cache Map, accessing an inner Map for each value in `keys`.
  If a value does not exist, return undefined.
  @param  {Map}        cache The base Map from which to get a value
  @param  {Array<Any>} keys  An array of keys for which nested Maps will be accessed
  @return {Any}        The value itself