assert(#Lines > 0)
  if s < 1 then s = 1 end
  if s > #Lines then s = #Lines end
  if e < 1 then e = 1 end
  if e > #Lines then e = #Lines end

                       Drawing.before = snapshot()

                       Drawing.before = snapshot(line_index-1, line_index)

                       save_to_disk(Lines, Filename)
                       record_undo_event({before=Drawing.before, after=snapshot(line_index-1, line_index+1)})