Insertion by one sometimes gives drammatically better results than

base/containers/flat_tree.h

Index: base/containers/flat_tree.h
diff --git a/base/containers/flat_tree.h b/base/containers/flat_tree.h
index 8e85f2377655f954c585be08bea9749edd79240c..5be811b047fc82c9d64176802f5b5f69c0ead5a0 100644
--- a/base/containers/flat_tree.h
+++ b/base/containers/flat_tree.h
@@ -200,6 +200,24 @@ class flat_tree {
   iterator insert(const_iterator position_hint, const value_type& x);
   iterator insert(const_iterator position_hint, value_type&& x);
 
+  template <typename I>
+  void insert_by_one(I first, I last) {
+    std::copy(first, last, std::inserter(*this, end()));
+  }
+
+  template <typename I>
+  void insert_merge(I first, I last) {
+    size_type original_size = size();
+    impl_.body_.insert(end(), first, last);
+    iterator new_elements = begin() + original_size;
+    std::stable_sort(new_elements, end(), value_compare());
+    std::inplace_merge(begin(), new_elements, end());
+    auto comparator = [this](const value_type& lhs, const value_type& rhs) {
+      return !value_comp()(lhs, rhs);
+    };
+    erase(std::unique(begin(), end(), comparator), end());
+  }
+
   template <class... Args>
   std::pair<iterator, bool> emplace(Args&&... args);