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diff --git a/src/library/scala/collection/mutable/ReusableBuilder.scala b/src/library/scala/collection/mutable/ReusableBuilder.scala
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+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2016, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+
+
+package scala
+package collection
+package mutable
+
+/** `ReusableBuilder` is a marker trait that indicates that a `Builder`
+ *  can be reused to build more than one instance of a collection.  In
+ *  particular, calling `result` followed by `clear` will produce a
+ *  collection and reset the builder to begin building a new collection
+ *  of the same type.
+ *
+ *  It is up to subclasses to implement this behavior, and to document any
+ *  other behavior that varies from standard `ReusableBuilder` usage
+ *  (e.g. operations being well-defined after a call to `result`, or allowing
+ *  multiple calls to result to obtain different snapshots of a collection under
+ *  construction).
+ *
+ *  @tparam  Elem  the type of elements that get added to the builder.
+ *  @tparam  To    the type of collection that it produced.
+ *
+ *  @since 2.12
+ */
+trait ReusableBuilder[-Elem, +To] extends Builder[Elem, To] {
+  /** Clears the contents of this builder.
+   *  After execution of this method, the builder will contain no elements.
+   *
+   *  If executed immediately after a call to `result`, this allows a new
+   *  instance of the same type of collection to be built.
+   */
+  override def clear(): Unit    // Note: overriding for Scaladoc only!
+
+  /** Produces a collection from the added elements.
+   *
+   *  After a call to `result`, the behavior of all other methods is undefined
+   *  save for `clear`.  If `clear` is called, then the builder is reset and
+   *  may be used to build another instance.
+   *
+   *  @return a collection containing the elements added to this builder.
+   */
+  override def result(): To    // Note: overriding for Scaladoc only!
+}