Assume `multiset<string> c; vector<string> v;`, then

    copy(v.begin(), v.end(), inserter(c, c.end()));

is OK.

    copy(v.begin(), v.end(), back_inserter(c));

is error. A `multiset` doesn't have `push_back` memeber function.

    copy(c.begin(), c.end(), inserter(v, v.end()));

is OK.

    copy(c.begin(), c.end(), back_inserter(v));

is OK.