scala> import scala.collection._
import scala.collection._

scala> val l = mutable.MutableList(1,2)
l: scala.collection.mutable.MutableList[Int] = MutableList(1, 2)

scala> val t = l.tail
t: scala.collection.mutable.MutableList[Int] = MutableList(2)

scala> l += 3
res0: l.type = MutableList(1, 2, 3)

// unnormal start here.....

scala> t.length
res3: Int = 1

// the t.length is 1, but why REPL print MutableList(2, 3)
scala> t
res1: scala.collection.mutable.MutableList[Int] = MutableList(2, 3)

// and t.toString print  MutableList(2)
scala> t.toString
res5: String = MutableList(2)