-- The surface may show the same file in multiple panes. Share line contents
  -- between such aliases (though editable panes may diverge).
  Cache = {}

end
function initialize_cache_if_necessary(id)
  if Cache[id] then return end
--?   print('init:', id)
  Cache[id] = {id=id, filename=Directory..id, left=0, right=Display_settings.column_width, lines={}, line_cache={}}
  load_from_disk(Cache[id])
  Links[id] = load_links(id)

  initialize_cache_if_necessary(id)

  result.lines = Cache[id].lines
  result.line_cache = deepcopy(Cache[id].line_cache)  -- should be tiny; deepcopy is just to eliminate any chance of aliasing

  load_from_disk(result)
  Links[id] = load_links(id)

  Text.redraw_all(result)

    process_all_links(x, function(target)
      initialize_cache_if_necessary(target)
      save_links(target)

    process_all_links(x, function(target_id)
      save_links(target_id)

  Cache[pane.id].lines = pane.lines

    initialize_cache_if_necessary(start_id)

    initialize_cache_if_necessary(start_id)

    initialize_cache_if_necessary(start_id)

  if pane.lines == Cache[pane.id].lines then
    -- break an alias
    pane.lines = deepcopy(pane.lines)
  end

  local pane = edit.initialize_state(0, 0, math.min(Display_settings.column_width, App.screen.width-Margin_right), Font_height, Line_height)
  pane.id = 'errors'
  pane.filename = Directory..pane.id
  pane.lines = Cache[pane.id].lines
  pane.line_cache = Cache[pane.id].line_cache  -- alias since we won't edit

  -- TODO: we might run into some bugs if we have multiple error panes visible
  -- on the surface; unlike everything else these currently alias.
  local pane = Error_log

  Text.redraw_all(pane)

  initialize_cache_if_necessary(id)

    load_from_disk(Cache[id])
    Display_settings.search_all_pane.lines = Cache[id].lines

    load_from_disk(Display_settings.search_all_pane)

    load_from_disk(Cache[id])
    Display_settings.search_all_pane.lines = Cache[id].lines

    load_from_disk(Display_settings.search_all_pane)

  Cache = {}

-- Precondition: both ids are already in Cache.

  if Links[start_id] == nil then
    Links[start_id] = load_links(start_id)
  end

  while Links[id][Opposite[rel]] do

  while true do
    if Links[id] == nil then
      Links[id] = load_links(id)
    end
    if Links[id][Opposite[rel]] == nil then
      break
    end

    initialize_cache_if_necessary(id)

    initialize_cache_if_necessary(curr)

    if Links[curr] == nil then
      Links[curr] = load_links(curr)
    end

    initialize_cache_if_necessary(curr_id)

    if Links[curr_id] == nil then
      Links[curr_id] = load_links(curr_id)
    end

  Cache[id] = {id=id, filename=Directory..id, lines={{mode='text', data=''}}, line_cache={{}}, left=0, right=Display_settings.column_width}

--?   outfile:write(json.encode(Links[id]))