private void PrevClick(object sender, EventArgs e)
        {
            TVRequestInfo header1 = _source.GetPrevious(ref _currFirstIndex);
            TVRequestInfo header2 = _source.GetPrevious(ref _currSecondIndex);

            _currFirstDiffIndex  = -1;
            _currSecondDiffIndex = -1;

            if (header1 != null && header2 != null)
            {
                if (!_buttonNext.Enabled)
                {
                    _buttonNext.Enabled = true;
                }

                DoDiff(_currFirstIndex, _currSecondIndex);
            }
            else
            {
                _buttonPrev.Enabled = false;
            }
        }