Bi-directional map in Scala

BiMap.scala

class BiMap[A, B] private(var a2b: Map[A, B], var b2a: Map[B, A]) extends Tuple2(a2b, b2a) with collection.mutable.MapLike[A, B, BiMap[A, B]] {
  override def +=(kv: (A, B)) = {
    a2b = a2b + kv
    b2a = b2a + kv.swap
    this
  }

  override def -=(a: A) = {
    get(a).foreach {b => b2a = b2a - b}
    a2b = a2b - a
    this
  }

  override def get(a: A) = a2b get a
  override def empty = BiMap.empty
  override def iterator = a2b.iterator
  override def seq = a2b.seq
}

object BiMap {
  def empty[A, B] = new BiMap(Map.empty[A, B], Map.empty[B, A])
}