/**
 * Chains over the entries of the `HashMap` using the specified function.
 *
 * **NOTE**: the hash and equal of both maps have to be the same.
 *
 * @tsplus static HashMap.Aspects flatMapWithIndex
 * @tsplus pipeable HashMap flatMapWithIndex
 */
export function flatMapWithIndex<K, V, A>(f: (k: K, v: V) => HashMap<K, A>) {
  return (self: HashMap<K, V>): HashMap<K, A> => {
    return self.reduceWithIndex(HashMap.empty<K, A>(), (z, k, v) =>
      z.mutate((m) => {
        f(k, v).forEachWithIndex((_k, _a) => {
          m.set(_k, _a)
        })
      }))
  }
}