fun <A> Arb.Companion.frequency(vararg arbs: Pair<Int, Arb<A>>): Arb<A> =
  arb(arbs.flatMap { it.second.edgecases() }) { rs ->
    require(arbs.isNotEmpty()) { "No Arb instances passed to Arb.frequency()." }

    val iters = arbs.toList().flatMap { (weight, arb) ->
      List(weight) { arb }
    }.map { it.values(rs).iterator() }

    fun next(): Sample<A>? {
      val iter = iters.shuffled(rs.random).first()
      return if (iter.hasNext()) iter.next() else null
    }

    sequence {
      while (true) {
        var next: Sample<A>? = null
        while (next == null)
          next = next()
        yield(next.value)
      }
    }
  }