\directlua{

      local current_instance  

      local custom_find_file =  custom_find_file or {}

      local mp_find_file_driver = function(name, mode, type) print("I am ",current_instance) if custom_find_file[current_instance] then return custom_find_file[current_instance](name, mode, type) else  return  "unregistered" end end 

      custom_find_file['mp1'] = function(name, mode, type) return name end

      custom_find_file['mp2'] = function(name, mode, type) if name == 'cmr10' then print'This should not happen' end return name end

 

      local mp1 = mplib.new{ 

        find_file = mp_find_file_driver

      }

      local mp2 = mplib.new{

          find_file = mp_find_file_driver

        }

      current_instance = "mp1"

      mp1:execute[[

        input cmr10;

      ]]

      current_instance = "mp2"

      mp2:execute[[

        input cmr10;

      ]]

    }

    \bye