for _,id in ipairs(Cache[src].links[rel]) do
      if id == target then
        add_error(('%s is already a %s of %s'):format(target, rel, src))
        return false

    if Cache[src].links[rel] then
      for _,id in ipairs(Cache[src].links[rel]) do
        if id == target then
          add_error(('%s is already a %s of %s'):format(target, rel, src))
          return false
        end