我想从现有的Set集合创建一个新的自定义Scala集合，稍后可以使用一些附加函数扩展该集合。
目前，我正尝试让它像标准集合Set一样工作。https://docs.scala-lang.org/overviews/core/custom-collections.html，其中他们有一个在Map上扩展的自定义集合。但是在Set上扩展似乎有点不同，我有大量的类型不兼容问题无法解决。
下面是我的启动代码：我不能定义的部分是addOne、substractOne、contains和iterator。

import scala.collection._

class TestSet[A]
  extends mutable.Set[A]
    with mutable.SetOps[A, mutable.Set, TestSet[A]] {

  override def empty: TestSet[A] = new TestSet

  // Members declared in scala.collection.mutable.Clearable
  override def clear(): Unit = immutable.Set.empty

  // Members declared in scala.collection.IterableOps
  override protected def fromSpecific(coll: IterableOnce[A]): TestSet[A] = TestSet.from(coll)

  override protected def newSpecificBuilder: mutable.Builder[A, TestSet[A]] = TestSet.newBuilder

  override def className = "TestSet"

  //override def subtractOne(elem: A): TestSet.this.type = ???

  //override def addOne(elem: A): TestSet.this.type = ???

  //override def contains(elem: A): Boolean = ???

   //override def iterator: Iterator[A] = {
   //}
}

object TestSet {
  def empty[A] = new TestSet[A]

  def from[A](source: IterableOnce[A]): TestSet[A] =
    source match {
      case pm: TestSet[A] => pm
      case _ => (newBuilder ++= source).result()
    }

  def apply[A](elem: A*): TestSet[A] = from(elem)

  def newBuilder[A]: mutable.Builder[A, TestSet[A]] =
    new mutable.GrowableBuilder[A, TestSet[A]](empty)

  import scala.language.implicitConversions

  implicit def toFactory[A](self: this.type): Factory[A, TestSet[A]] =
    new Factory[A, TestSet[A]] {
      def fromSpecific(it: IterableOnce[A]): TestSet[A] = self.from(it)

      def newBuilder: mutable.Builder[A, TestSet[A]] = self.newBuilder
    }

}