Merge two Boost Fusion maps by key

I have two boost::fusion::maps that I want to merge in a certain way. Fro the two maps I want to generate a third that has all the keys present in both maps and the values are added if both a present. For example:

#include <boost/fusion/container/map.hpp>
namespace bfn = boost::fusion;
using namespace boost::fusion;

struct x{};
struct y{};
struct z{};

int main(){

    auto m = make_map<x, y>(2, 4.3);
    auto n = make_map<x>(2.);
    auto l = accumulate_merge(m, n); // how this function should look like?
}

After that l will be equivalent to make_map<x, y>(2 + 2., 4.3).

I have no clue where to start. I tried to begin with join (and the eliminate duplicates but I got complicated pretty fast).

Is there a tool in Boost Fusion that can help me?

(There are lots of subtleties like what to do if for the same key the two corresponding types are different --but still addable--. But any first version will help).