template<class _FwdIt,
    class _Ty,
    class _Diff> inline
    _FwdIt _Lower_bound(_FwdIt _First, _FwdIt _Last, const _Ty& _Val, _Diff *)
    {   // find first element not before _Val, using operator<
    _Diff _Count = 0;
    _Distance(_First, _Last, _Count);

    while (0 < _Count)
        {   // divide and conquer, find half that contains answer
        _Diff _Count2 = _Count / 2;
        _FwdIt _Mid = _First;
        _STD advance(_Mid, _Count2);

        if (*_Mid < _Val)
//      if (_DEBUG_LT(*_Mid, _Val))
            {   // try top half
            _First = ++_Mid;
            _Count -= _Count2 + 1;
            }
        else
            _Count = _Count2;
        }
    return (_First);
    }