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diff --git a/boost/histogram/accumulators/weighted_sum.hpp b/boost/histogram/accumulators/weighted_sum.hpp
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+// Copyright 2015-2018 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)
+
+#ifndef BOOST_HISTOGRAM_ACCUMULATORS_WEIGHTED_SUM_HPP
+#define BOOST_HISTOGRAM_ACCUMULATORS_WEIGHTED_SUM_HPP
+
+#include <boost/histogram/fwd.hpp>
+#include <type_traits>
+
+namespace boost {
+namespace histogram {
+namespace accumulators {
+
+/// Holds sum of weights and its variance estimate
+template <typename RealType>
+class weighted_sum {
+public:
+  weighted_sum() = default;
+  explicit weighted_sum(const RealType& value) noexcept
+      : sum_of_weights_(value), sum_of_weights_squared_(value) {}
+  weighted_sum(const RealType& value, const RealType& variance) noexcept
+      : sum_of_weights_(value), sum_of_weights_squared_(variance) {}
+
+  /// Increment by one.
+  weighted_sum& operator++() { return operator+=(1); }
+
+  /// Increment by value.
+  template <typename T>
+  weighted_sum& operator+=(const T& value) {
+    sum_of_weights_ += value;
+    sum_of_weights_squared_ += value * value;
+    return *this;
+  }
+
+  /// Added another weighted sum.
+  template <typename T>
+  weighted_sum& operator+=(const weighted_sum<T>& rhs) {
+    sum_of_weights_ += static_cast<RealType>(rhs.sum_of_weights_);
+    sum_of_weights_squared_ += static_cast<RealType>(rhs.sum_of_weights_squared_);
+    return *this;
+  }
+
+  /// Scale by value.
+  weighted_sum& operator*=(const RealType& x) {
+    sum_of_weights_ *= x;
+    sum_of_weights_squared_ *= x * x;
+    return *this;
+  }
+
+  bool operator==(const RealType& rhs) const noexcept {
+    return sum_of_weights_ == rhs && sum_of_weights_squared_ == rhs;
+  }
+
+  template <typename T>
+  bool operator==(const weighted_sum<T>& rhs) const noexcept {
+    return sum_of_weights_ == rhs.sum_of_weights_ &&
+           sum_of_weights_squared_ == rhs.sum_of_weights_squared_;
+  }
+
+  template <typename T>
+  bool operator!=(const T& rhs) const noexcept {
+    return !operator==(rhs);
+  }
+
+  /// Return value of the sum.
+  const RealType& value() const noexcept { return sum_of_weights_; }
+
+  /// Return estimated variance of the sum.
+  const RealType& variance() const noexcept { return sum_of_weights_squared_; }
+
+  // lossy conversion must be explicit
+  template <class T>
+  explicit operator T() const {
+    return static_cast<T>(sum_of_weights_);
+  }
+
+  template <class Archive>
+  void serialize(Archive&, unsigned /* version */);
+
+private:
+  RealType sum_of_weights_ = RealType();
+  RealType sum_of_weights_squared_ = RealType();
+};
+
+} // namespace accumulators
+} // namespace histogram
+} // namespace boost
+
+#ifndef BOOST_HISTOGRAM_DOXYGEN_INVOKED
+namespace std {
+template <class T, class U>
+struct common_type<boost::histogram::accumulators::weighted_sum<T>,
+                   boost::histogram::accumulators::weighted_sum<U>> {
+  using type = boost::histogram::accumulators::weighted_sum<common_type_t<T, U>>;
+};
+
+template <class T, class U>
+struct common_type<boost::histogram::accumulators::weighted_sum<T>, U> {
+  using type = boost::histogram::accumulators::weighted_sum<common_type_t<T, U>>;
+};
+
+template <class T, class U>
+struct common_type<T, boost::histogram::accumulators::weighted_sum<U>> {
+  using type = boost::histogram::accumulators::weighted_sum<common_type_t<T, U>>;
+};
+} // namespace std
+#endif
+
+#endif