order: ...
  - fix sort() which needed to adjust locations after each value move
  - provide 'ordered' pipelet an order downstream pipelet base class which handles
    the complexity of editing a stateful dependent pipelet using three methods
    provided by the implementor:
      insert_( at  , value  )
      remove_( from, value  )
      update_( from, update )

    These methods are supposed to update the state of a stateful pipelet according
    to an ordered source. The first parameter is the position in the set, and the
    second parameter is the value to insert, or remove, or an update containing
    the old and new values but for which the position did not change.