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diff --git a/src/library/scala/collection/generic/MutableMapBuilder.scala b/src/library/scala/collection/generic/MutableMapBuilder.scala
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+++ b/src/library/scala/collection/generic/MutableMapBuilder.scala
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+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2009, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+
+// $Id: ListBuffer.scala 14378 2008-03-13 11:39:05Z dragos $
+
+package scala.collection.generic
+
+// import collection.immutable.{List, Nil, ::}
+
+/** The canonical builder for collections that are addable, i.e. that support an efficient + method
+ *  which adds an element to the collection.
+ *  Collections are built from their empty element using this + method.
+ *  @param empty   The empty element of the collection.
+ */
+class MutableMapBuilder[A, B, Coll <: mutable.Map[A, B] with MapTemplate[A, B, Coll]](empty: Coll)
+extends Builder[(A, B), Coll, Any] {
+  protected var elems: Coll = empty
+  def +=(x: (A, B)) { elems += x }
+  def clear() { elems = empty }
+  def result: Coll = elems
+}