gistfile1.txt

interface Persistence
  persist

Persister implements Persistence
  
object Foo
  internal value A
  internal value B

  persistVia Persister
    KeyValueHash 'A' => A, 'B' => B
    Persister persist KeyValueHash

--

Foo doesnt expose internal state representation except via a generic agreed upon mechanism for anything like display, persist etc. Actual what for persist, display is handled by other objects. They just have a standard interface for accomplishing it which Foo can implement. There is no 'behavior' for Foo except how it interacts with objects that do things.