set的可变与不可变情况```scala def main(args: Array[String]): Unit = {

def main(args: Array[String]): Unit = {
  // 分：可变set，不可变
  // 默认情况下：使用的set就是不可变的

  //可变的set
  val set1 = scala.collection.mutable.Set("apple","grape","pear")
  set1 += "banana"

  println(set1)

  //不可变的set
  val course = scala.collection.immutable.Set("apple","banana")
  //报错：coure += “xxx“
  var newCourse = course + "pear"
  println(newCourse)
}