scala> import scala.collection.mutable
import scala.collection.mutable
scala> val buf = mutable.ListBuffer[Int]()
buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer()
scala> object o {
     |   val xs = List(1, 2, 3)
     | }
defined module o
scala> import o._
import o._
scala> buf += xs
<console>:11: error: type mismatch:
 found   : scala.collection.immutable.List[Int](o.xs)
 required: String
buf += xs
       ^
<console>:11: error: type mismatch:
 found   : String
 required: scala.collection.mutable.ListBuffer[Int]
buf += xs
^
scala> buf ++= xs
res1: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 3)
scala> :quit