clipp.h

     */
    bool scoped() const noexcept
    {
        return scoped_;
    }


    //---------------------------------------------------------------
    /** @brief determines if children are mutually exclusive alternatives */
    group& exclusive(bool yes) {
        exclusive_ = yes;
        return *this;
    }
    /** @brief returns if children are mutually exclusive alternatives */
    bool exclusive() const noexcept {
        return exclusive_;
    }


    //---------------------------------------------------------------
    /** @brief returns true, if any child is required to match */
    bool any_required() const
    {
        return std::any_of(children_.begin(), children_.end(),
            [](const child& n){ return n.required(); });
    }
    /** @brief returns true, if all children are required to match */
    bool all_required() const
    {
        return std::all_of(children_.begin(), children_.end(),
            [](const child& n){ return n.required(); });
    }


    //---------------------------------------------------------------
    /** @brief returns true if any child is optional (=non-required) */
    bool any_optional() const {
        return !all_required();
    }
    /** @brief returns true if all children are optional (=non-required) */
    bool all_optional() const {
        return !any_required();
    }


    //---------------------------------------------------------------
    /** @brief returns if the entire group is blocking / positional */
    bool blocking() const noexcept {
        return token<group>::blocking() || (exclusive() && all_blocking());
    }
    //-----------------------------------------------------
    /** @brief determines if the entire group is blocking / positional */
    group& blocking(bool yes) {
        return token<group>::blocking(yes);
    }

    //---------------------------------------------------------------
    /** @brief returns true if any child is blocking */
    bool any_blocking() const
    {
        return std::any_of(children_.begin(), children_.end(),
            [](const child& n){ return n.blocking(); });
    }
    //---------------------------------------------------------------
    /** @brief returns true if all children is blocking */
    bool all_blocking() const
    {
        return std::all_of(children_.begin(), children_.end(),
            [](const child& n){ return n.blocking(); });
    }


    //---------------------------------------------------------------
    /** @brief returns if any child is a value parameter (recursive) */
    bool any_flagless() const
    {
        return std::any_of(children_.begin(), children_.end(),
            [](const child& p){
                return p.is_param() && p.as_param().flags().empty();
            });
    }
    /** @brief returns if all children are value parameters (recursive) */
    bool all_flagless() const
    {
        return std::all_of(children_.begin(), children_.end(),
            [](const child& p){
                return p.is_param() && p.as_param().flags().empty();
            });
    }


    //---------------------------------------------------------------
    /** @brief adds child parameter at the end */
    group&
    push_back(const parameter& v) {
        children_.emplace_back(v);
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child parameter at the end */
    group&
    push_back(parameter&& v) {
        children_.emplace_back(std::move(v));
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child group at the end */
    group&
    push_back(const group& g) {
        children_.emplace_back(g);
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child group at the end */
    group&
    push_back(group&& g) {
        children_.emplace_back(std::move(g));
        return *this;
    }


    //-----------------------------------------------------
    /** @brief adds children (groups and/or parameters) */
    template<class Param1, class Param2, class... Params>
    group&
    push_back(Param1&& param1, Param2&& param2, Params&&... params)
    {
        children_.reserve(children_.size() + 2 + sizeof...(params));
        push_back(std::forward<Param1>(param1));
        push_back(std::forward<Param2>(param2), std::forward<Params>(params)...);
        return *this;
    }


    //---------------------------------------------------------------
    /** @brief adds child parameter at the beginning */
    group&
    push_front(const parameter& v) {
        children_.emplace(children_.begin(), v);
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child parameter at the beginning */
    group&
    push_front(parameter&& v) {
        children_.emplace(children_.begin(), std::move(v));
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child group at the beginning */
    group&
    push_front(const group& g) {
        children_.emplace(children_.begin(), g);
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds child group at the beginning */
    group&
    push_front(group&& g) {
        children_.emplace(children_.begin(), std::move(g));
        return *this;
    }


    //---------------------------------------------------------------
    /** @brief adds all children of other group at the end */
    group&
    merge(group&& g)
    {
        children_.insert(children_.end(),
                      std::make_move_iterator(g.begin()),
                      std::make_move_iterator(g.end()));
        return *this;
    }
    //-----------------------------------------------------
    /** @brief adds all children of several other groups at the end */
    template<class... Groups>
    group&
    merge(group&& g1, group&& g2, Groups&&... gs)
    {
        merge(std::move(g1));
        merge(std::move(g2), std::forward<Groups>(gs)...);
        return *this;
    }


    //---------------------------------------------------------------
    /** @brief indexed, nutable access to child */
    child& operator [] (size_type index) noexcept {
        return children_[index];
    }
    /** @brief indexed, non-nutable access to child */
    const child& operator [] (size_type index) const noexcept {
        return children_[index];
    }

    //---------------------------------------------------------------
    /** @brief mutable access to first child */
          child& front()       noexcept { return children_.front(); }
    /** @brief non-mutable access to first child */
    const child& front() const noexcept { return children_.front(); }
    //-----------------------------------------------------
    /** @brief mutable access to last child */
          child& back()       noexcept { return children_.back(); }
    /** @brief non-mutable access to last child */
    const child& back() const noexcept { return children_.back(); }


    //---------------------------------------------------------------
    /** @brief returns true, if group has no children, false otherwise */
    bool empty() const noexcept     { return children_.empty(); }

    /** @brief returns number of children */
    size_type size() const noexcept { return children_.size(); }

    /** @brief returns number of nested levels; 1 for a flat group */
    size_type depth() const {
        size_type n = 0;
        for(const auto& c : children_) {
            auto l = 1 + c.depth();
            if(l > n) n = l;
        }
        return n;
    }


    //---------------------------------------------------------------
    /** @brief returns mutating iterator to position of first element */
          iterator  begin()       noexcept { return children_.begin(); }
    /** @brief returns non-mutating iterator to position of first element */
    const_iterator  begin() const noexcept { return children_.begin(); }
    /** @brief returns non-mutating iterator to position of first element */
    const_iterator cbegin() const noexcept { return children_.begin(); }

    /** @brief returns mutating iterator to position one past the last element */
          iterator  end()         noexcept { return children_.end(); }
    /** @brief returns non-mutating iterator to position one past the last element */
    const_iterator  end()   const noexcept { return children_.end(); }
    /** @brief returns non-mutating iterator to position one past the last element */
    const_iterator cend()   const noexcept { return children_.end(); }


    //---------------------------------------------------------------
    /** @brief returns augmented iterator for depth first searches
     *  @details taverser knows end of iteration and can skip over children
     */
    depth_first_traverser
    begin_dfs() const noexcept {
        return depth_first_traverser{*this};
    }


    //---------------------------------------------------------------
    /** @brief returns recursive parameter count */
    size_type param_count() const {
        size_type c = 0;
        for(const auto& n : children_) {
            c += n.param_count();
        }
        return c;
    }


    //---------------------------------------------------------------
    /** @brief returns range of all flags (recursive) */
    arg_list all_flags() const
    {
        std::vector<arg_string> all;
        gather_flags(children_, all);
        return all;
    }

    /** @brief returns true, if no flag occurs as true
     *         prefix of any other flag (identical flags will be ignored) */
    bool flags_are_prefix_free() const
    {
        const auto fs = all_flags();

        using std::begin; using std::end;
        for(auto i = begin(fs), e = end(fs); i != e; ++i) {
            if(!i->empty()) {
                for(auto j = i+1; j != e; ++j) {
                    if(!j->empty() && *i != *j) {
                        if(i->find(*j) == 0) return false;
                        if(j->find(*i) == 0) return false;
                    }
                }
            }
        }

        return true;
    }


    //---------------------------------------------------------------
    /** @brief returns longest common prefix of all flags */
    arg_string common_flag_prefix() const
    {
        arg_list prefixes;
        gather_prefixes(children_, prefixes);
        return str::longest_common_prefix(prefixes);
    }


private:
    //---------------------------------------------------------------
    static void
    gather_flags(const children_store& nodes, arg_list& all)
    {
        for(const auto& p : nodes) {
            if(p.is_group()) {
                gather_flags(p.as_group().children_, all);
            }
            else {
                const auto& pf = p.as_param().flags();
                using std::begin;
                using std::end;
                if(!pf.empty()) all.insert(end(all), begin(pf), end(pf));
            }
        }
    }
    //---------------------------------------------------------------
    static void
    gather_prefixes(const children_store& nodes, arg_list& all)
    {
        for(const auto& p : nodes) {
            if(p.is_group()) {
                gather_prefixes(p.as_group().children_, all);
            }
            else if(!p.as_param().flags().empty()) {
                auto pfx = str::longest_common_prefix(p.as_param().flags());
                if(!pfx.empty()) all.push_back(std::move(pfx));
            }
        }
    }

    //---------------------------------------------------------------
    children_store children_;
    bool exclusive_ = false;
    bool joinable_ = false;
    bool scoped_ = false;
};



/*************************************************************************//**
 *
 * @brief group or parameter
 *
 *****************************************************************************/
using pattern = group::child;



/*************************************************************************//**
 *
 * @brief makes a group of parameters and/or groups
 *
 *****************************************************************************/
inline group
operator , (parameter a, parameter b)
{
    return group{std::move(a), std::move(b)}.scoped(false);
}

//---------------------------------------------------------
inline group
operator , (parameter a, group b)
{
    return !b.scoped() && !b.blocking() && !b.exclusive() && !b.repeatable()
        && !b.joinable() && (b.doc().empty() || b.doc() == a.doc())
       ? b.push_front(std::move(a))
       : group{std::move(a), std::move(b)}.scoped(false);
}

//---------------------------------------------------------
inline group
operator , (group a, parameter b)
{
    return !a.scoped() && !a.blocking() && !a.exclusive() && !a.repeatable()
        && !a.joinable() && (a.doc().empty() || a.doc() == b.doc())
       ? a.push_back(std::move(b))
       : group{std::move(a), std::move(b)}.scoped(false);
}

//---------------------------------------------------------
inline group
operator , (group a, group b)
{
    return !a.scoped() && !a.blocking() && !a.exclusive() && !a.repeatable()
        && !a.joinable() && (a.doc().empty() || a.doc() == b.doc())
       ? a.push_back(std::move(b))
       : group{std::move(a), std::move(b)}.scoped(false);
}



/*************************************************************************//**
 *
 * @brief makes a group of alternative parameters or groups
 *
 *****************************************************************************/
template<class Param, class... Params>
inline group
one_of(Param param, Params... params)
{
    return group{std::move(param), std::move(params)...}.exclusive(true);
}


/*************************************************************************//**
 *
 * @brief makes a group of alternative parameters or groups
 *
 *****************************************************************************/
inline group
operator | (parameter a, parameter b)
{
    return group{std::move(a), std::move(b)}.scoped(false).exclusive(true);
}

//-------------------------------------------------------------------
inline group
operator | (parameter a, group b)
{
    return !b.scoped() && !b.blocking() && b.exclusive() && !b.repeatable()
        && !b.joinable()
        && (b.doc().empty() || b.doc() == a.doc())
        ? b.push_front(std::move(a))
        : group{std::move(a), std::move(b)}.scoped(false).exclusive(true);
}

//-------------------------------------------------------------------
inline group
operator | (group a, parameter b)
{
    return !a.scoped() && a.exclusive() && !a.repeatable() && !a.joinable()
        && a.blocking() == b.blocking()
        && (a.doc().empty() || a.doc() == b.doc())
        ? a.push_back(std::move(b))
        : group{std::move(a), std::move(b)}.scoped(false).exclusive(true);
}

inline group
operator | (group a, group b)
{
    return !a.scoped() && a.exclusive() &&!a.repeatable() && !a.joinable()
        && a.blocking() == b.blocking()
        && (a.doc().empty() || a.doc() == b.doc())
        ? a.push_back(std::move(b))
        : group{std::move(a), std::move(b)}.scoped(false).exclusive(true);
}



namespace detail {

inline void set_blocking(bool) {}

template<class P, class... Ps>
void set_blocking(bool yes, P& p, Ps&... ps) {
    p.blocking(yes);
    set_blocking(yes, ps...);
}

} // namespace detail


/*************************************************************************//**
 *
 * @brief makes a parameter/group sequence by making all input objects blocking
 *
 *****************************************************************************/
template<class Param, class... Params>
inline group
in_sequence(Param param, Params... params)
{
    detail::set_blocking(true, param, params...);
    return group{std::move(param), std::move(params)...}.scoped(true);
}


/*************************************************************************//**
 *
 * @brief makes a parameter/group sequence by making all input objects blocking
 *
 *****************************************************************************/
inline group
operator & (parameter a, parameter b)
{
    a.blocking(true);
    b.blocking(true);
    return group{std::move(a), std::move(b)}.scoped(true);
}

//---------------------------------------------------------
inline group
operator & (parameter a, group b)
{
    a.blocking(true);
    return group{std::move(a), std::move(b)}.scoped(true);
}

//---------------------------------------------------------
inline group
operator & (group a, parameter b)
{
    b.blocking(true);
    if(a.all_blocking() && !a.exclusive() && !a.repeatable() && !a.joinable()
        && (a.doc().empty() || a.doc() == b.doc()))
    {
        return a.push_back(std::move(b));
    }
    else {
        if(!a.all_blocking()) a.blocking(true);
        return group{std::move(a), std::move(b)}.scoped(true);
    }
}

inline group
operator & (group a, group b)
{
    if(!b.all_blocking()) b.blocking(true);
    if(a.all_blocking() && !a.exclusive() && !a.repeatable()
        && !a.joinable() && (a.doc().empty() || a.doc() == b.doc()))
    {
        return a.push_back(std::move(b));
    }
    else {
        if(!a.all_blocking()) a.blocking(true);
        return group{std::move(a), std::move(b)}.scoped(true);
    }
}



/*************************************************************************//**
 *
 * @brief makes a group of parameters and/or groups
 *        where all single char flag params ("-a", "b", ...) are joinable
 *
 *****************************************************************************/
inline group&
joinable(group& param) {
    return param.joinable(true);
}

inline group&&
joinable(group&& param) {
    return std::move(param.joinable(true));
}

//-------------------------------------------------------------------
template<class... Params>
inline group
joinable(parameter param, Params... params)
{
    return group{std::move(param), std::move(params)...}.joinable(true);
}

template<class P2, class... Ps>
inline group
joinable(group p1, P2 p2, Ps... ps)
{
    return group{std::move(p1), std::move(p2), std::move(ps)...}.joinable(true);
}

template<class Param, class... Params>
inline group
joinable(doc_string docstr, Param param, Params... params)
{
    return group{std::move(param), std::move(params)...}
                .joinable(true).doc(std::move(docstr));
}



/*************************************************************************//**
 *
 * @brief makes a repeatable copy of a parameter
 *
 *****************************************************************************/
inline parameter
repeatable(parameter p) {
    return p.repeatable(true);
}

/*************************************************************************//**
 *
 * @brief makes a repeatable copy of a group
 *
 *****************************************************************************/
inline group
repeatable(group g) {
    return g.repeatable(true);
}



/*************************************************************************//**
 *
 * @brief makes a group of parameters and/or groups
 *        that is repeatable as a whole
 *        Note that a repeatable group consisting entirely of non-blocking
 *        children is equivalent to a non-repeatable group of
 *        repeatable children.
 *
 *****************************************************************************/
template<class P2, class... Ps>
inline group
repeatable(parameter p1, P2 p2, Ps... ps)
{
    return group{std::move(p1), std::move(p2),
                 std::move(ps)...}.repeatable(true);
}

template<class P2, class... Ps>
inline group
repeatable(group p1, P2 p2, Ps... ps)
{
    return group{std::move(p1), std::move(p2),
                 std::move(ps)...}.repeatable(true);
}



/*************************************************************************//**
 *
 * @brief recursively prepends a prefix to all flags
 *
 *****************************************************************************/
inline parameter&&
with_prefix(const arg_string& prefix, parameter&& p) {
    return std::move(with_prefix(prefix, p));
}


//-------------------------------------------------------------------
inline group&
with_prefix(const arg_string& prefix, group& params)
{
    for(auto& p : params) {
        if(p.is_group()) {
            with_prefix(prefix, p.as_group());
        } else {
            with_prefix(prefix, p.as_param());
        }
    }
    return params;
}


inline group&&
with_prefix(const arg_string& prefix, group&& params)
{
    return std::move(with_prefix(prefix, params));
}


template<class Param, class... Params>
inline group
with_prefix(arg_string prefix, Param&& param, Params&&... params)
{
    return with_prefix(prefix, group{std::forward<Param>(param),
                                     std::forward<Params>(params)...});
}



/*************************************************************************//**
 *
 * @brief recursively prepends a prefix to all flags
 *
 * @param shortpfx : used for single-letter flags
 * @param longpfx  : used for flags with length > 1
 *
 *****************************************************************************/
inline parameter&&
with_prefixes_short_long(const arg_string& shortpfx, const arg_string& longpfx,
                         parameter&& p)
{
    return std::move(with_prefixes_short_long(shortpfx, longpfx, p));
}


//-------------------------------------------------------------------
inline group&
with_prefixes_short_long(const arg_string& shortFlagPrefix,
                         const arg_string& longFlagPrefix,
                         group& params)
{
    for(auto& p : params) {
        if(p.is_group()) {
            with_prefixes_short_long(shortFlagPrefix, longFlagPrefix, p.as_group());
        } else {
            with_prefixes_short_long(shortFlagPrefix, longFlagPrefix, p.as_param());
        }
    }
    return params;
}


inline group&&
with_prefixes_short_long(const arg_string& shortFlagPrefix,
                         const arg_string& longFlagPrefix,
                         group&& params)
{
    return std::move(with_prefixes_short_long(shortFlagPrefix, longFlagPrefix,
                                              params));
}


template<class Param, class... Params>
inline group
with_prefixes_short_long(const arg_string& shortFlagPrefix,
                         const arg_string& longFlagPrefix,
                         Param&& param, Params&&... params)
{
    return with_prefixes_short_long(shortFlagPrefix, longFlagPrefix,
                                    group{std::forward<Param>(param),
                                          std::forward<Params>(params)...});
}








/*************************************************************************//**
 *
 * @brief parsing implementation details
 *
 *****************************************************************************/
namespace detail {


/*************************************************************************//**
 *
 * @brief DFS traverser that keeps track of 'scopes'
 *        scope = all parameters that are either bounded by
 *        two blocking parameters on the same depth level
 *        or the beginning/end of the outermost group
 *
 *****************************************************************************/
class scoped_dfs_traverser
{
public:
    using dfs_traverser = group::depth_first_traverser;

    scoped_dfs_traverser() = default;

    explicit
    scoped_dfs_traverser(const group& g):
        pos_{g}, lastMatch_{}, posAfterLastMatch_{}, scopes_{},
        curMatched_{false}, ignoreBlocks_{false},
        repeatGroupStarted_{false}, repeatGroupContinues_{false}
    {}

    const dfs_traverser& base() const noexcept { return pos_; }
    const dfs_traverser& last_match() const noexcept { return lastMatch_; }

    const group& parent() const noexcept { return pos_.parent(); }
    const group* repeat_group() const noexcept { return pos_.repeat_group(); }
    const group* join_group() const noexcept { return pos_.join_group(); }

    const pattern* operator ->() const noexcept { return pos_.operator->(); }
    const pattern& operator *() const noexcept { return *pos_; }

    const pattern* ptr() const noexcept { return pos_.operator->(); }

    explicit operator bool() const noexcept { return bool(pos_); }

    bool joinable() const noexcept { return pos_.joinable(); }
    arg_string common_flag_prefix() const { return pos_.common_flag_prefix(); }

    void ignore_blocking(bool yes) { ignoreBlocks_ = yes; }

    void invalidate() { pos_.invalidate(); curMatched_ = false; }
    bool matched() const noexcept { return curMatched_; }

    bool start_of_repeat_group() const noexcept { return repeatGroupStarted_; }

    //-----------------------------------------------------
    scoped_dfs_traverser&
    next_sibling() { pos_.next_sibling(); return *this; }

    scoped_dfs_traverser&
    next_alternative() { pos_.next_alternative(); return *this; }

    scoped_dfs_traverser&
    next_after_siblings() { pos_.next_after_siblings(); return *this; }

    //-----------------------------------------------------
    scoped_dfs_traverser&
    operator ++ ()
    {
        if(!pos_) return *this;

        if(pos_.is_last_in_path()) {
            return_to_outermost_scope();
            return *this;
        }

        //current pattern can block if it didn't match already
        if(!ignoreBlocks_ && !matched()) {
            //current group can block if we didn't have any match in it
            if(pos_.is_last_in_group() && pos_.parent().blocking()
                && (!posAfterLastMatch_ || &(posAfterLastMatch_.parent()) != &(pos_.parent())))
            {
                //ascend to parent's level
                ++pos_;
                //skip all siblings of parent group
                pos_.next_after_siblings();
                if(!pos_) return_to_outermost_scope();
            }
            else if(pos_->blocking() && !pos_->is_group()) {
                if(pos_.parent().exclusive()) { //is_alternative(pos_.level())) {
                    pos_.next_alternative();
                } else {
                    //no match => skip siblings of blocking param
                    pos_.next_after_siblings();
                }
                if(!pos_) return_to_outermost_scope();
            } else {
                ++pos_;
            }
        } else {
            ++pos_;
        }
        check_left_scope();
        return *this;
    }

    //-----------------------------------------------------
    void next_after_match(scoped_dfs_traverser match)
    {
        if(!match || ignoreBlocks_) return;

        check_repeat_group_start(match);

        lastMatch_ = match.base();

        if(!match->blocking() && match.base().parent().blocking()) {
            match.pos_.back_to_parent();
        }

        //if match is not in current position & current position is blocking
        //=> current position has to be advanced by one so that it is
        //no longer reachable within current scope
        //(can happen for repeatable, blocking parameters)
        if(match.base() != pos_ && pos_->blocking()) pos_.next_sibling();

        if(match->blocking()) {
            if(match.pos_.is_alternative()) {
                //discard other alternatives
                match.pos_.skip_alternatives();
            }

            if(is_last_in_current_scope(match.pos_)) {
                //if current param is not repeatable -> back to previous scope
                if(!match->repeatable() && !match->is_group()) {
                    curMatched_ = false;
                    pos_ = std::move(match.pos_);
                    if(!scopes_.empty()) pos_.undo(scopes_.top());
                }
                else { //stay at match position
                    curMatched_ = true;
                    pos_ = std::move(match.pos_);
                }
            }
            else { //not last in current group
                //if current param is not repeatable, go directly to next
                if(!match->repeatable() && !match->is_group()) {
                    curMatched_ = false;
                    ++match.pos_;
                } else {
                    curMatched_ = true;
                }

                if(match.pos_.level() > pos_.level()) {
                    scopes_.push(pos_.undo_point());
                    pos_ = std::move(match.pos_);
                }
                else if(match.pos_.level() < pos_.level()) {
                    return_to_level(match.pos_.level());
                }
                else {
                    pos_ = std::move(match.pos_);
                }
            }
            posAfterLastMatch_ = pos_;
        }
        else {
            if(match.pos_.level() < pos_.level()) {
                return_to_level(match.pos_.level());
            }
            posAfterLastMatch_ = pos_;
        }
        repeatGroupContinues_ = repeat_group_continues();
    }

private:
    //-----------------------------------------------------
    bool is_last_in_current_scope(const dfs_traverser& pos)
    {
        if(scopes_.empty()) return pos.is_last_in_path();
        //check if we would leave the current scope on ++
        auto p = pos;
        ++p;
        return p.level() < scopes_.top().level();
    }

    //-----------------------------------------------------
    void check_repeat_group_start(const scoped_dfs_traverser& newMatch)
    {
        const auto newrg = newMatch.repeat_group();
        if(!newrg) {
            repeatGroupStarted_ = false;
        }
        else if(lastMatch_.repeat_group() != newrg) {
            repeatGroupStarted_ = true;
        }
        else if(!repeatGroupContinues_ || !newMatch.repeatGroupContinues_) {
            repeatGroupStarted_ = true;
        }
        else {
            //special case: repeat group is outermost group
            //=> we can never really 'leave' and 'reenter' it
            //but if the current scope is the first element, then we are
            //conceptually at a position 'before' the group
            repeatGroupStarted_ = scopes_.empty() || (
                    newrg == pos_.root() &&
                    scopes_.top().param() == &(*pos_.root()->begin()) );
        }
        repeatGroupContinues_ = repeatGroupStarted_;
    }

    //-----------------------------------------------------
    bool repeat_group_continues()
    {
        if(!repeatGroupContinues_) return false;
        const auto curRepGroup = pos_.repeat_group();
        if(!curRepGroup) return false;
        if(curRepGroup != lastMatch_.repeat_group()) return false;
        if(!posAfterLastMatch_) return false;
        return true;
    }

    //-----------------------------------------------------
    void check_left_scope()
    {
        if(posAfterLastMatch_) {
            if(pos_.level() < posAfterLastMatch_.level()) {
                while(!scopes_.empty() && scopes_.top().level() >= pos_.level()) {
                    pos_.undo(scopes_.top());
                    scopes_.pop();
                }
                posAfterLastMatch_.invalidate();
            }
        }
        while(!scopes_.empty() && scopes_.top().level() > pos_.level()) {
            pos_.undo(scopes_.top());
            scopes_.pop();
        }
        repeatGroupContinues_ = repeat_group_continues();
    }

    //-----------------------------------------------------
    void return_to_outermost_scope()
    {
        posAfterLastMatch_.invalidate();

        if(scopes_.empty()) {
            pos_.invalidate();
            repeatGroupContinues_ = false;
            return;
        }

        while(!scopes_.empty() && (!pos_ || pos_.level() >= 1)) {
            pos_.undo(scopes_.top());
            scopes_.pop();
        }
        while(!scopes_.empty()) scopes_.pop();

        repeatGroupContinues_ = repeat_group_continues();
    }

    //-----------------------------------------------------
    void return_to_level(int level)
    {
        if(pos_.level() <= level) return;
        while(!scopes_.empty() && pos_.level() > level) {
            pos_.undo(scopes_.top());
            scopes_.pop();
        }
    };

    dfs_traverser pos_;