title: What does "++=" mean in Scala
body: <p>This is the <a href="https://github.com/scala/scala/blob/v2.10.3/src/library/scala/collection/TraversableLike.scala" rel="noreferrer">implementation</a> of <code>flatMap</code> in Scala</p>
  
  <pre>
  def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    def builder = bf(repr) // ...
    val b = builder
    for (x &lt;- this) b ++= f(x).seq
    b.result
  }
  </pre>
  
  <p>What does <code>++=</code> mean here ?</p>