Squashed 'third_party/immer/' content from commit ad3e3556d

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extra/python/example.py

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+#!/usr/bin/env python##
+
+# immer: immutable data structures for C++
+# Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+#
+# This software is distributed under the Boost Software License, Version 1.0.
+# See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+
+
+# include:intro/start
+import immer
+
+v0 = immer.Vector().append(13).append(42)
+assert v0[0] == 13
+assert v0[1] == 42
+assert len(v0) == 2
+
+v1 = v0.set(0, 12)
+assert v0.tolist() == [13, 42]
+assert v1.tolist() == [12, 42]
+# include:intro/end