template <class branch, class branches, class extends = searchert<branch> >
class breadth_first_searcht: public brancht<branch, branches, extends> {
public:
    breadth_first_searcht() {}
    breadth_first_searcht(branch* v) { this->search(v); }
    virtual void search(branch* v) {
        if (v) {
            do {
                if (this->found(v)) { break; }
                for (auto b: v->branches()) {
                    this->queue_branch(b);
                }
            } while ((v = this->pop_branch()));
        }
    }
};