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diff --git a/src/boost/libs/histogram/examples/guide_parallel_filling.cpp b/src/boost/libs/histogram/examples/guide_parallel_filling.cpp
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+++ b/src/boost/libs/histogram/examples/guide_parallel_filling.cpp
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+// Copyright 2018-2019 Hans Dembinski
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+//[ guide_parallel_filling
+
+#include <boost/histogram.hpp>
+#include <boost/histogram/algorithm/sum.hpp>
+#include <cassert>
+#include <functional>
+#include <thread>
+#include <vector>
+
+// dummy fill function, to be executed in parallel by several threads
+template <typename Histogram>
+void fill(Histogram& h) {
+  for (unsigned i = 0; i < 1000; ++i) { h(i % 10); }
+}
+
+int main() {
+  using namespace boost::histogram;
+
+  /*
+    Create histogram with container of thread-safe counters for parallel filling in
+    several threads. Only filling is thread-safe, other guarantees are not given.
+  */
+  auto h = make_histogram_with(dense_storage<accumulators::thread_safe<unsigned>>(),
+                               axis::integer<>(0, 10));
+
+  /*
+    Run the fill function in parallel from different threads. This is safe when a
+    thread-safe accumulator and a storage with thread-safe cell access are used.
+  */
+  auto fill_h = [&h]() { fill(h); };
+  std::thread t1(fill_h);
+  std::thread t2(fill_h);
+  std::thread t3(fill_h);
+  std::thread t4(fill_h);
+  t1.join();
+  t2.join();
+  t3.join();
+  t4.join();
+
+  // Without a thread-safe accumulator, this number may be smaller.
+  assert(algorithm::sum(h) == 4000);
+}
+
+//]