集合的操作 /用于添加或去除元素的操作符  

val digits  = Set(1,7,8,0)
  val a= digits contains 0
  System.out.println(a)
   val b = Set(1,7).subsetOf(digits)
  System.out.println(b)
 val primes = Set(2,3,5,7)
 val u = digits union primes
  u.foreach(System.out.println(_))
val u1 = digits & primes
u1.foreach(System.out.println(_))
val u2 = digits -- primes
u2.foreach(System.out.println(_))