vToBeDeleted := Self.InternalRange(vBeginning, vEnd);
  
  { optimized cases: adjacent to the gap. }
  {$IFDEF OPT}
  if (vToBeDeleted[0].End1 = fGapStart) and (vToBeDeleted[0].End1 > vToBeDeleted[0].Beginning) { TODO and some gap size limit is not exceeded } then begin
    fGapStart := vToBeDeleted[0].Beginning;
    assert(fGapStart = vBeginning);
    FillChar(fBuffer[fGapStart], vToBeDeleted[0].End1 - fGapStart, 0); { for better debugging. }
    vToBeDeleted[0].End1 := fGapStart;
  end;

  if (vToBeDeleted[1].Beginning = fGapEnd) and (vToBeDeleted[1].End1 > vToBeDeleted[1].Beginning) { TODO and some gap size limit is not exceeded } then begin
    fGapEnd := vToBeDeleted[1].End1;
    assert(fGapEnd = vEnd);
    FillChar(fBuffer[vToBeDeleted[1].Beginning], fGapEnd - vToBeDeleted[1].Beginning, 0); { for better debugging. }
    vToBeDeleted[1].Beginning := fGapEnd;
  end;
  {$ENDIF}

  { normal case: need to move data around. }

  vFrontier := Self.PositionFromTextMark(fEnd1);

  vEnd := Self.PositionFromTextMark(aPosition, aCount); // where the block to be deleted ends.
  vSource := Self.InternalRange(vEnd, vFrontier);
  
  vToBeDeleted := Self.InternalRange(vBeginning, vFrontier); // as copy destination, use _everything_ until the end.

  // vSource -> vToBeDeleted.
  // tail -> move to vEnd (beginning of vSource).

  Dump(Format('Copy %d', [fEnd1.Position - (aPosition.Position + aCount)]));
  Self.Copy(vSource, vToBeDeleted, fEnd1.Position - (aPosition.Position + aCount));