template <typename C, typename N>>
C<N> sqrts(const C<N> &xs) {
    C<N> r;
    remove_copy_if(xs.begin(), xs.end(), back_inserter(r), [](N i) { return i >= 0; }
    transform(r.begin(), r.end(), r.begin(), [](N i) { sqrt(i); });
    return r;
}