-- links might contain either a single target or a list of them
function process_all_links(x, fn)
  if type(x) == 'string' then
    fn(x)
  else
    for _,target in ipairs(x) do
      fn(target)
    end
  end
end

  -- TODO: non-unique
  for rel,target in pairs(Cache[start_id].links) do
    local pane = load_pane(target)
    add_title(pane, rel)
    table.insert(column, pane)

  for rel,x in pairs(Cache[start_id].links) do
    process_all_links(x, function(target)
      local pane = load_pane(target)
      add_title(pane, rel)
      table.insert(column, pane)
    end)

-- links might contain either a single target or a list of them
function process_all_links(x, fn)
  if type(x) == 'string' then
    fn(x)
  else
    for _,target in ipairs(x) do
      fn(target)
    end
  end
end