Mail::Message::DB
	new Mail::Message::DB { ml_name => , seq => }
	add( $msg )
	get($table, $key)
	set($table, $key, 4value)

		thread_summary($key)

 $msg ---> Mail::Message::ToHTML
                |     A
                |     |
        new()   |     |
    add($msg)   |     | thread_summary() 
    set()       |     | get()
                |     |
                V     |
            Mail::Message::DB =========== tie() ===> AnyDBM
            A      + update .db files via tie()
  add($msg) |      + thread analyze
            | 
            |      +===============================> cache with time limit
            |      ||
            |      ||
            FML::UDB ? << unified cache database interface >>
              A
              |
          FML::*::* modules



  $msg  Mail::Message OBJECT
  ===>  method using tie()
  --->  method not Tie::*