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void assign( K const& keyBegin, K const& keyEnd, V const& val ) {
        // body of assign() function
        if (!(keyBegin < keyEnd)) return;

        std::pair<K,V> beginExtra;
        std::pair<K,V> endExtra;
        bool beginHasExtra = false;
        bool endHasExtra = false;
// open range for iterator
        typename std::map<K,V>::iterator itBegin;
        itBegin = m_map.lower_bound(keyBegin);
        if ( itBegin!=m_map.end() && keyBegin < itBegin->first ) {
            if (itBegin != m_map.begin()) {
                beginHasExtra = true;
                --itBegin;
                beginExtra = std::make_pair(itBegin->first, itBegin->second);
            }
            
        }
// close range for iterator
        typename std::map<K,V>::iterator itEnd;
        itEnd = m_map.lower_bound(keyEnd);
        if ( itEnd!=m_map.end() && keyEnd < itEnd->first ) {
            endHasExtra = true;
            typename std::map<K,V>::iterator extraIt = itEnd;
            --extraIt;
            endExtra = std::make_pair(keyEnd, extraIt->second);
        }

        // four canonical conflicts
        bool insertMid = true;
        if (beginHasExtra) {
            if (beginExtra.second == val)
                insertMid = false;
        } else {
            if (itBegin != m_map.begin()) {
                typename std::map<K,V>::iterator beforeMid = itBegin;
                --beforeMid;
                if (beforeMid->second == val)
                    insertMid = false;
            }
        }


        if (endHasExtra) {
            if ( (insertMid && endExtra.second == val) || (!insertMid && endExtra.second == beginExtra.second) )
                endHasExtra = false;
        } else {
            if ( (insertMid && itEnd!=m_map.end() && itEnd->second == val) || (!insertMid && itEnd!=m_map.end() && itEnd->second == beginExtra.second) )
                itEnd = m_map.erase(itEnd);
        }

        itBegin = m_map.erase(itBegin, itEnd);
        if (beginHasExtra)
            itBegin = m_map.insert(itBegin, beginExtra);
        if (insertMid)
            itBegin = m_map.insert(itBegin, std::make_pair(keyBegin, val));
        if (endHasExtra)
            m_map.insert(itBegin, endExtra);
  }