accumulateとreduceを相互に関連項目として記載

Author: faithandbrave

reference/numeric/accumulate.md

@@ -161,6 +161,10 @@ T accumulate(InputIterator first, InputIterator last, T init,
 * std::move[link /reference/utility/move.md]
 
 
+## 関連項目
+- [`std::reduce()`](reduce.md)
+
+
 ## 参照
 - [P0616R0 De-pessimize legacy `<numeric>` algorithms with `std::move`](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0616r0.pdf)
 - [P1645R1 `constexpr` for `<numeric>` algorithms](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1645r1.html)

reference/numeric/reduce.md

@@ -159,6 +159,10 @@ product : 120
 - [Visual C++](/implementation.md#visual_cpp): ??
 
 
+## 関連項目
+- [`std::accumulate()`](accumulate.md)
+
+
 ## 参照
 - [P0024R2 The Parallelism TS Should be Standardized](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0024r2.html)
 - [P0467R2 Iterator Concerns for Parallel Algorithms](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0467r2.html)