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diff --git a/lib/pure/collections/rtarrays.nim b/lib/pure/collections/rtarrays.nim
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+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2015 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+
+## Module that implements a fixed length array whose size
+## is determined at runtime. Note: This is not ready for other people to use!
+##
+## Unstable API.
+
+const
+  ArrayPartSize = 10
+
+type
+  RtArray*[T] = object ##
+    L: Natural
+    spart: seq[T]
+    apart: array[ArrayPartSize, T]
+
+template usesSeqPart(x): untyped = x.L > ArrayPartSize
+
+proc initRtArray*[T](len: Natural): RtArray[T] =
+  result.L = len
+  if usesSeqPart(result):
+    newSeq(result.spart, len)
+
+proc getRawData*[T](x: var RtArray[T]): ptr UncheckedArray[T] =
+  if usesSeqPart(x): cast[ptr UncheckedArray[T]](addr(x.spart[0]))
+  else: cast[ptr UncheckedArray[T]](addr(x.apart[0]))
+
+#proc len*[T](x: RtArray[T]): int = x.L
+