- On creation of an intersection point, it should be added to the
  lists of POIs of the intersecting objects. The user should be able
  to inhibit this. I am unsure how he should be able to inihibit it
  specifically for some of the objects. Maybe I won't allow
  this. Anyway, it is planned to let objects ignore points. It can
  always be done very specifically this way.

- update strategy
  - points buffer their position
  - whenever an object changes something that affects downstream
    objects it notifies the construction object
  - the construction object saves the lowest order index that has
    changes
  - whenever a point would like to access its buffered value it asks
    the construction object if it is still uptodate
  - if the order index of the asking object is higher than the last
    change then a refresh is triggered; either a refresh of the
    required range (that would require a list of changed objects, -
    not just the lowest one) or a full refresh (that would probably
    not be too expensive)
  - a refresh can be executed by just updating all objects
    sequentially since an object can only depend on objects with -
    lower order indices (I hope this is really true, but I cannot -
    come up with a scenario where this restriction would be too -
    harsh).

- Think about a role shared by Point/DerivedPoint.
