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diff --git a/test/benchmarks/src/main/scala/benchmark/KeySeq.scala b/test/benchmarks/src/main/scala/benchmark/KeySeq.scala
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+++ b/test/benchmarks/src/main/scala/benchmark/KeySeq.scala
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+package benchmark
+
+/** A sequence of keys.
+  * 
+  * Tests of maps and sets require a sequence of keys that can be used
+  * to add entries and possibly to find them again.
+  * This type provides such a sequence.
+  * 
+  * Note that this needn't be a "sequence" in the full sense of [[collection.Seq]],
+  * particularly in that it needn't extend [[PartialFunction]].
+  * 
+  * @tparam K the type of the keys
+  */
+trait KeySeq[K] {
+  /** Selects a key by its index in the sequence.
+    * Repeated calls with the same index return the same key (by reference equality).
+    * 
+    * @param idx The index to select. Should be non-negative and less than `size`.
+    */
+  def apply(idx: Int): K
+
+  /** The size of this sequence. */
+  def size: Int
+}