How to `parallel_reduce` a C++ map key or value?

I don't find it from google. How can I parallel_reduce either the key or value of a map? For example:

       map<T, size_t> counts;
	size_t count = parallel_reduce(
		blocked_range<pair<T, size_t>>(0, counts.size()), 0,
		[&](tbb::blocked_range<pair<T, size_t>> const &r, size_t running_total)
		{
			for (auto it = r.begin(); it != r.end(); it++)
				running_total += it->second;
			return running_total;
		},
		std::plus<pair<T, size_t>>());

从 issue 中的 C++ 示例开始，确定针对 map 的键或值 intended 使用哪种 parallel_reduce 操作。一个有用的结果应说明所示 counts map 的正确 reduction 形式，并明确所需的结果类型和组合类型。